[media] pxa_camera: fix the buffer free path

Fix the error path where the video buffer wasn't allocated nor
mapped. In this case, in the driver free path don't try to unmap memory
which was not mapped in the first place.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>