V4L/DVB (11663): Fix a warning introduced by git commit ec5f5bf8

/home/v4l/master/v4l/radio-si470x.c: In function 'si470x_fops_release':
/home/v4l/master/v4l/radio-si470x.c:1218: warning: label 'unlock' defined but not used

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>