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

This reverts commit 747dba7d.

It breaks concurrent vbi and video capturing:
While v4l2->users is the number of users of the whole device (all device nodes),
v4l2_fh_is_singular() only checks the number of users of a specific device node.
As a result. if one device node is open and a second device node is opened
(closed), the device is reinitialized (streaming is stopped).

Reported-by: Hans Verkuil <hans.verkuil@cisco.com>
Tested-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>