[media] em28xx-v4l: get rid of field "users" in struct em28xx_v4l2

Instead of counting the number of opened file handles, use function
v4l2_fh_is_singular_file() in em28xx_v4l2_open() and em28xx_v4l2_close() to
determine if the file handle is the first/last opened one.

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