V4L/DVB (5691): Ov7670: reset clkrc in rgb565 mode

A bug in the ov7670 sensor causes it to introduce noise unless the CLKRC
register is rewritten *after* setting the image mode.  Naturally,
resetting CLKRC in this way will cause other modes to fail.  So
carefully poke the register only when indicated.

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