firewire: fix return code

Fix this warning on x86-64

drivers/firewire/fw-cdev.c:798: warning: initialization from incompatible pointer type

by making the return code of ioctl_send_request() the same as all the
other ioctl_xxx() return codes.

Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>