[media] rtl2832_sdr: clamp bandwidth to nearest legal value in automode

Clamp bandwidth to nearest legal value in automode in order to pass
v4l2-compliance test.

Reported-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>