Fix several bugs in the diagnostic path in mpr_user.c.

o In mpr_post_fw_diag_buffer() and mpr_release_fw_diag_buffer(),
  check the reply to see whether it is NULL.  It can be NULL (and
  the command non-NULL) if the controller gets reinitialized
  while we're waiting for the command to complete but the
  driver structures aren't reallocated.  The driver structures
  generally won't be reallocated unless there is a firmware upgrade
  that changes one of the IOCFacts.

o When freeing diagnostic buffers in mpr_diag_register() and
  mpr_diag_unregister(), zero/NULL out the buffer after freeing it.
  This will prevent a duplicate free in some situations.

Fix a couple of busdma issues in the mpr(4) driver.

In mpr_diag_register(), which is used to register diagnostic
buffers with the mpr(4) firmware, we allocate DMAable memory.

There were several issues here:
 o No checking of the bus_dmamap_load() return value.  If the load
   failed or got deferred, mpr_diag_register() continued on as if
   nothing had happened.  We now check the return value and bail
   out if it fails.

 o No waiting for a deferred load callback.  bus_dmamap_load()
   calls a supplied callback when the mapping is done.  This is
   generally done immediately, but it can be deferred.
   mpr_diag_register() did not check to see whether the callback
   was already done before proceeding on.  We now sleep until the
   callback is done if it is deferred.

 o No call to bus_dmamap_sync(... BUS_DMASYNC_PREREAD) after the
   memory is allocated and loaded.  This is necessary on some
   platforms to synchronize host memory that is going to be updated
   by a device.

mprvar.h:
        Add a new structure, struct mpr_busdma_context, that is
        used for deferred busdma load callbacks.

        Add a prototype for mpr_memaddr_wait_cb().
mpr.c:
        Add a new busdma callback function, mpr_memaddr_wait_cb().
        This provides synchronization for callers that want to
        wait on a deferred bus_dmamap_load() callback.

mpr_user.c:
        In bus_dmamap_register(), add a call to bus_dmamap_sync()
        with the BUS_DMASYNC_PREREAD flag set after an allocation
        is loaded.

        Also, check the return value of bus_dmamap_load().  If it
        fails, bail out.  If it is EINPROGRESS, wait for the
        callback to happen.  We use an interruptible sleep (msleep
        with PCATCH) and let the callback clean things up if we get
        interrupted.

Fix a return value in mpr_diag_register().

In the case of a failure to create a busdma tag, we were returning
ENOMEM instead of MPR_DIAG_FAILURE.  The caller expects an MPR_
status returned, not an errno.