Fix panic in firewire and creation of invalid config ROM.

sys/boot/i386/libfirewire/firewire.c:
sys/dev/firewire/firewire.c:
Fix configuration ROM generation count wrapping logic
so that the generation count is never outside of
allowed limits (0x2 -> 0xF).

sys/dev/firewire/firewire.c:
In fw_xfer_unload(), xfer->fc may be NULL. Protect
against this before taking the fc lock.

Submitted by: gibbs
MFC after: 1 week
Sponsored by: Spectra Logic
MFSpectraBSD: 1110685 on 2015/01/05