[media] via-camera: Fix OLPC serial check

The code that checks the OLPC serial port is never built at the moment,
because CONFIG_OLPC_XO_1_5 doesn't exist and probably won't be added.

Fix it so that it gets compiled in, only executes on OLPC laptops, and
move the check into the probe routine.

The compiler is smart enough to eliminate this code when CONFIG_OLPC=n
(due to machine_is_olpc() always returning false).

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