[media] tda18271: prog_cal and prog_tab variables should be s32, not u8

Fix type of prog_cal and prog_tab variables to avoid any possible
calculation errors.

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