media: dvb: mb86a16: check the return value of mb86a16_read()

return an error code if mb86a16_read() gets an unexpected return
value.

Signed-off-by: Yuanjun Gong <ruc_gongyuanjun@163.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil: add {} around the else statement]