[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>
diff --git a/drivers/media/dvb-frontends/cx22700.c b/drivers/media/dvb-frontends/cx22700.c
index f2a90f9..3d399d9 100644
--- a/drivers/media/dvb-frontends/cx22700.c
+++ b/drivers/media/dvb-frontends/cx22700.c
@@ -139,7 +139,7 @@
 	if (p->code_rate_HP == FEC_4_5 || p->code_rate_LP == FEC_4_5)
 		return -EINVAL;
 
-	if (p->guard_interval < GUARD_INTERVAL_1_32 ||
+	if ((int)p->guard_interval < GUARD_INTERVAL_1_32 ||
 	    p->guard_interval > GUARD_INTERVAL_1_4)
 		return -EINVAL;
 
@@ -152,7 +152,7 @@
 	    p->modulation != QAM_64)
 		return -EINVAL;
 
-	if (p->hierarchy < HIERARCHY_NONE ||
+	if ((int)p->hierarchy < HIERARCHY_NONE ||
 	    p->hierarchy > HIERARCHY_4)
 		return -EINVAL;