V4L/DVB (12869): tda18271: fix comments and make tda18271_agc debug less verbose

Don't display "no agc configuration provided" unless DBG_ADV is set.

Fix comments in function, tda18271_agc.  This config variable is not
for LNA configuration -- it is for external AGC configuration.

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