firewire: don't panic on invalid AR request buffer

BUG() at this place is wrong.  (Unless if the low level driver would
already do higher-level input validation of incoming request headers.)

Invalid incoming requests or bugs in the controller which corrupt the
AR-req buffer needlessly crashed the box because this is run in tasklet
context.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>