media: saa7146: fix error logic at saa7146_vv_init()

As the first thing this function does is to call
v4l2_device_register(), it should call v4l2_device_unregister()
if an error occurs, the same way as done at saa7146_vv_release().

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>