V4L/DVB (13111): tda18271: more signedness fixes

Convert tda18271_rf_tracking_filter_cal.rf_[ab][12] from int to s32.
Convert tda18271_priv.tm_rfcal from unsigned int to u8.
Cast subtractions between u32 values as s32.

Signed-off-by: Michael Krufky <mkrufky@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>