[media] rtl28xxu: return demod reg page from driver cache

Return current active rtl2830/rtl2832 register page from the driver
cache in order to reduce I2C I/O. Register page is already cached
due to I2C write needs.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>