media: dvb-frontends/rtl2832.c: fix missing error code

Fixes this smatch warning:

drivers/media/dvb-frontends/rtl2832.c:702 rtl2832_read_status() warn: missing error code 'ret'

'ret' is indeed not set, so set it to -EINVAL so a proper error
code is returned.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>