media: dvb_frontend: Add unlocked_ioctl in dvb_frontend.c

Adds unlocked ioctl function directly in dvb_frontend.c instead of using
dvb_generic_ioctl().

Signed-off-by: Jaedon Shin <jaedon.shin@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>