V4L/DVB (8034): tda18271: fix IF notch frequency handling

The IF notch bit gets unset when we update the Main Post Div register
value, before we have a chance to write the desired IF notch setting
to the tuner.  Move the IF notch configuration to after we update MPD.

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