pxa_camera: remove init() callback

pxa_camera init() callback is sometimes abused to setup MFP for PXA CIF, or
even to request GPIOs to be used by the camera *sensor*. These initializations
can be performed statically in machine init functions.

The current semantics for this init() callback is ambiguous anyways, it is
invoked in pxa_camera_activate(), hence at device node open, but its users use
it like a generic initialization to be done at module init time (configure
MFP, request GPIOs for *sensor* control).

Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>