media: common: saa7146: allow S_STD(G_STD)

If the requested TV standard is identical to the current
TV standard, then return 0, even when the vb2 queues are
busy.

This fixes a V4L2 compliance issue.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>