[media] dib8000: Fix sub-channel range

isdbt_sb_subchannel is unsigned with 8 bits. So, it will
never be -1. Instead, any value bigger than 13 is invalid.

As is, the current code generates the following warnings:

drivers/media/dvb-frontends/dib8000.c: In function 'dib8000_set_isdbt_common_channel':
drivers/media/dvb-frontends/dib8000.c:2358:3: warning: comparison is always true due to limited range of data type [-Wtype-limits]
drivers/media/dvb-frontends/dib8000.c: In function 'dib8000_tune':
drivers/media/dvb-frontends/dib8000.c:3107:8: warning: comparison is always false due to limited range of data type [-Wtype-limits]
drivers/media/dvb-frontends/dib8000.c:3153:9: warning: comparison is always false due to limited range of data type [-Wtype-limits]
drivers/media/dvb-frontends/dib8000.c:3160:5: warning: comparison is always false

It should also be noticed that ARIB STD-B31, item
"3.15.6.8 Number of segments" at TMCC table defines the
value 15 for unused segment, and 14 as reserved.

So, better to change the check to consider any value
bigger than 13 to mean that sub-channels should be
disabled, fixing the warning and doing the right thing
even if an invalid value is filled by userspace.

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