[media] rtl2832: change the i2c gate to be mux-locked

The root i2c adapter lock is then no longer held by the i2c mux during
accesses behind the i2c gate, and such accesses need to take that lock
just like any other ordinary i2c accesses do.

So, declare the i2c gate mux-locked, and zap the regmap overrides
that makes the i2c accesses unlocked and use plain old regmap
accesses. This also removes the need for the regmap wrappers used by
rtl2832_sdr, so deconvolute the code further and provide the regmap
handle directly instead of the wrapper functions.

Tested-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>