Fix a kernel panic when unloading isp(4).

In the current implementation, the isp_kthread() threads never exit.

The target threads do have an exit mode from isp_attach(), but it is
not invoked from isp_detach().

Ensure isp_detach() notifies threads started for each channel, such
that they exit before their parent device softc detaches, and thus
before the module does. Otherwise, a page fault panic occurs later in:

sysctl_kern_proc

sysctl_out_proc
  kern_proc_out
    fill_kinfo_proc
      fill_kinfo_thread
        strlcpy(kp->ki_wmesg, td->td_wmesg, sizeof(kp->ki_wmesg));

For isp_kthread() (and isp(4) target threads), td->td_wmesg references
now-unmapped memory after the module has been unloaded. These threads
are typically msleep()ing at the time of unload, but they could also
attempt to execute now-unmapped code segments.

MFC after: 1 month
Sponsored by: Spectra Logic
MFSpectraBSD: r1070921 on 2014/06/22 13:01:17