[media] xc5000: allow drivers to set desired firmware in xc5000_attach

newer versions of the xc5000 silicon require newer firmware
while remaining 100% driver compatible.  original versions
of the xc5000a continue to use the same firmware.

Signed-off-by: Michael Krufky <mkrufky@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>