[media] cx25821: remove bogus btcx_risc dependency

Those btcx_risc functions are meant for use with bttv, other drivers
should just allocate the memory themselves.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>