[media] tda18271: fix bad calculation of main post divider byte

R_MPD bit 3 does not depend on analog vs. digital. Just use
(0x7f & pd) directly from the values in the main pll table.

Thanks to Stefan Sibiga for pointing this out.

Cc: Stefan Sibiga <stefansibiga@yahoo.ca>
Signed-off-by: Michael Krufky <mkrufky@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>