V4L/DVB (12372): saa7134: fix lock imbalance

There seems to be one superfluos unlock in a poll function. Remove it.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>