V4L/DVB (6978): tda18271: store frequency and bandwidth after successful tune

Store last tuned frequency & bandwidth after successful tune.

Clean up tune functions -- remove pointer to tune function in
state structure, instead call tune function based on priv->id.

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