[media] rtl2832: implement delayed I2C gate close

Delay possible I2C gate close a little bit in order to see if there
is next message coming to tuner in a sequence.

Also, export private muxed I2C adapter. That is aimed only for SDR
extension module as SDR belongs to same RTL2832 physical I2C bus (it
is physically property of RTL2832, whilst it is own kernel module).

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