[media] rtl2830: move statistics to read_status()

Move statistics polling to read_status() in order to avoid use of
kernel work. Also replace home made sign extension used for
statistics with kernel sign_extend32().

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>