V4L/DVB (13076): xc5000: make the definition of the FM input part of the xc5000 config struct

Remove hard-coded definition of the xc5000 FM radio input, making it a
parameter passed in when doing the attach call.

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>