[media] rtl2832_sdr: fix Kconfig dependencies

MEDIA_SDR_SUPPORT and I2C_MUX are needed for rtl2832_sdr.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>