V4L/DVB (7027): tda18271: put the device in standby mode during sleep()

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