[media] em28xx-v4l: get rid of struct em28xx_fh

struct em28xx_fh isn't needed anymore because the only used field which is left is struct v4l2_fh fh.
Use struct v4l2_fh directly and remvove struct em28xx_fh.

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