firewire: make PCI device id constant

The id_table field of the struct pci_driver is constant in <linux/pci.h>
so it is worth to make pci_table also constant.  Found with Coccinelle.

Signed-off-by: Márton Németh <nm127@freemail.hu>
Cc: Julia Lawall <julia@diku.dk>
Cc: cocci@diku.dk
Signed-off-by: Stefan Richter stefanr@s5r6.in-berlin.de> (changelog)