firewire: NULL check on malloc in fw_busreset()

[guest-seuros]

Summary

fw_busreset() allocates newrom with M_NOWAIT from interrupt context.
If the allocation fails, crom_load() dereferences a NULL pointer.

Skip the config ROM comparison on allocation failure so the next bus
reset will retry.

Test Plan

Hard to reproduce reliably. Triggered by wiggling the cable on a
battery-powered FireWire camera, the intermittent connection causes
rapid bus resets that race with allocation, eventually hitting the
NULL path. Panic no longer occurs with the fix.