Ensure that we only call the busdma unmap/flush routines once, when
the buffer is being freed.

• When buffers are cloned, the original mapping isn't copied but it wasn't freeing the mapping until later. To be safe, free the mapping when the buffer is cloned.

• ath_freebuf() now no longer calls the busdma sync/unmap routines.

• ath_tx_freebuf() now calls sync/unmap.

• Call sync first, before calling unmap.

Tested:

• AR5416, STA mode