[media] dvb-frontends: get rid of some "always false" warnings

On gcc, enums are generally unsigned, except if a negative value
is declared. Due to that, warnings may happen there:
drivers/media/dvb-frontends/cx22700.c:142:2: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
drivers/media/dvb-frontends/cx22700.c:155:2: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
drivers/media/dvb-frontends/cx24123.c:341:2: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
drivers/media/dvb-frontends/l64781.c:183:2: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
drivers/media/dvb-frontends/l64781.c:187:2: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
drivers/media/dvb-frontends/mt312.c:552:2: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
drivers/media/dvb-frontends/mt312.c:560:2: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
As other compilers might be using signed values, the better is to
keep the checks there, casting the value to avoid the warning.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>