[media] ov7670: allow configuration of image size, clock speed, and I/O method

These parameters need to be configurable based on the host system.
They can now be communicated through the s_config call.

The old CONFIG_OLPC_XO_1 selector was not correct; this kind of
arrangement wouldn't allow for a universal kernel that would work on both
laptops.

Certain parts of the probe routine had to be moved later (into s_config),
because we can't do any I/O until we know which I/O method has been
selected through this mechanism.

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