V4L/DVB (5834): dvb-core: fix signedness warnings and const stripping

Make some pointers const, and then delete some now unnecessary casts,
which were the wrong signedness anyway, being used to strip the const
from another pointer.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>