media: firewire: Make structure fdtv_ca constant

Static structure fdtv_ca, of type dvb_device, is only used when it is
passed as the third argument to function dvb_register_device(). However,
in the definition of dvb_register_device, the third parameter is
declared as constant. Hence make fdtv_ca const as well.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>