V4L/DVB (7434): tda18271: set rfagc modes during channel configuration

Set rfagc to high speed mode before setting the pll.
Set rfagc to normal speed mode at the end of the function.

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