V4L/DVB (12203): radio-si470x: fix lock imbalance

There is one path with omitted unlock in si470x_fops_release. Fix that.

Cc: Tobias Lorenz <tobias.lorenz@gmx.net>
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>