media: coda: fix macroblocks count control usage

Commit b2d3bef1 ("media: coda: Add a V4L2 user for control error
macroblocks count") add the control for the decoder devices. But
during streamon() this ioctl gets called for all (encoder and decoder)
devices and on encoder devices this causes a null pointer exception.

Fix this by setting the control only if it is really accessible.

Fixes: b2d3bef1

 ("media: coda: Add a V4L2 user for control error macroblocks count")
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Cc: <stable@vger.kernel.org>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>