[media] em28xx-audio: fix user counting in snd_em28xx_capture_open()

dev->adev.users always needs to be increased when snd_em28xx_capture_open() is
called and succeeds.

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