V4L/DVB (13987): [STV090x] Quit processing if the tuner did not lock

Exit stv090x_algo() if the tuner did not lock. This might happen due to
missing signal or invalid/incomplete tuning parameters.

Signed-off-by: Oliver Endriss <o.endriss@gmx.de>
Signed-off-by: Manu Abraham <abraham.manu@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>