[media] cx24120: do not allow an invalid delivery system types

cx24120_set_frontend currently allows invalid delivery system types
other than SYS_DVBS2 and SYS_DVBS.  Fix this by returning -EINVAL
for invalid values.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Jemma Denson <jdenson@gmail.com>