media: dvb-frontends: mb86a16.c: always use the same error path

If the message length was wrong, the dprintk() after the 'err' label
was bypassed. Fix that, and fix a smatch warning at the same time:

mb86a16.c:1514 mb86a16_send_diseqc_msg() warn: missing unwind goto?

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