[media] em28xx-v4l: do not call em28xx_init_camera() if the device has no sensor

This avoids the unnecessary temporary registration of a dummy V4L2 clock.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>