V4L/DVB (6907): tda18271: create separate calc_pll functions

Consolidate duplicated code by creating functions:

tda18271_calc_main_pll
tda18271_calc_cal_pll

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>