media: am437x-vpfe: Setting STD to current value is not an error

VIDIOC_S_STD should not return an error if the value is identical
to the current one.
This error was highlighted by the v4l2-compliance test.

Signed-off-by: Benoit Parrot <bparrot@ti.com>
Acked-by: Lad Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>