Fix one cause of firewire panics.

sys/dev/firewire/firewire.c:
In fw_xfer_unload(), clear the FWXF_INQ flag on the
xfer under protection of the FW_GMTX, after the
xfer is removeed from the tx/rx queue. Otherwise
it is possible for the xfer to be removed again
(corrupting the list or immediately panicing) from
another thread that has found this xfer in the
transaction label table.

Submitted by: gibbs
MFC after: 1 week
Sponsored by: Spectra Logic
MFSpectraBSD: 1110200 on 2015/01/02