firewire: fw-core: local variable shadows a global one

Sparse warned about it although it was apparently harmless:

drivers/firewire/fw-cdev.c:624:23: warning: symbol 'interrupt' shadows an earlier one
include/asm/hw_irq.h:29:13: originally declared here

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