firewire: core: typecast from gfp_t to bool more safely

An idr related patch introduced the following sparse warning:
  drivers/firewire/core-cdev.c:488:33: warning: incorrect type in initializer (different base types)
  drivers/firewire/core-cdev.c:488:33:    expected bool [unsigned] [usertype] preload
  drivers/firewire/core-cdev.c:488:33:    got restricted gfp_t
So let's convert from gfp_t bitfield to Boolean explicitly and safely.

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