Fix a FWXF_INQ race in the firewire driver.

sys/dev/firewire/firewire.c:
In fw_xfer_unload() expand lock coverage so that
the test for FWXF_INQ doesn't race with it being
cleared in another thread.

Submitted by: gibbs
MFC after: 1 week
Sponsored by: Spectra Logic
MFSpectraBSD: 1110207 on 2015/01/02