[media] em28xx-v4l: simplify some pointers in em28xx_init_camera()

Pointer v4l2" can be used instead of "dev->v4l2, which saves some characters.

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