[media] em28xx: fix implementation of s_stream

On em28xx driver, s_stream subdev ops was not implemented
properly. It was used only to disable stream, never enabling it.
That was the root cause of the regression when we added support
for s_stream on tvp5150 driver.

With that, we can get rid of the changes on tvp5150 side,
e. g. changeset 47de9bf8

 ('[media] tvp5150: Fix breakage for serial usage').

Tested video output on em2820+tvp5150 on WinTV USB2 and
video and/or vbi output on em288x+tvp5150 on HVR 950.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>