V4L/DVB (12202): em28xx, fix lock imbalance

There is one omitted unlock in em28xx_usb_probe. Fix that.

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>