V4L/DVB (8318): OV7670: don't reject unsupported settings

For VIDIOC_G_FMT/VIDIOC_TRY_FMT, the V4L2 API spec states:
"Drivers should not return an error code unless the input is ambiguous"
"Very simple, inflexible devices may even ignore all input and always
return the default parameters."
"When the requested buffer type is not supported drivers return an
EINVAL error code."
i.e. returning errors for unsupported fields is bad, and it's ok to
unconditionally overwrite user-requested settings

This patch makes ov7670 meet that behaviour, and brings it in line with
other drivers e.g. stk-webcam. It also fixes compatibility with (unpatched)
gstreamer.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>