[media] xc4000: added firmware_name parameter

The firmware_name module parameter makes it possible to set the firmware
file name. It defaults to "xc4000.fw" if not specified.

Signed-off-by: Istvan Varga <istvan_v@mailbox.hu>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>