Add function, tda18271_set_standby_mode.

During sleep, enter standby mode with slave tuner output enabled,
loop through on and xtal oscillator on.
Signed-off-by: NMichael Krufky <mkrufky@linuxtv.org>
Signed-off-by: NMauro Carvalho Chehab <mchehab@infradead.org>