[media] rtl2832: disable regmap register cache

Caching register reads causes some random I/O errors on channel
change. Disable caching now in order to avoid those errors.

Reverts partly commit dcadb82a



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