[media] em28xx: fix error path in em28xx_start_analog_streaming()

Increase the streaming_users count only if streaming start succeeds.

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