[media] as3645a: move relevant code under __devinit/__devexit

There is no needs to keep .remove under .exit.text. This driver is for a
standalone chip that could be on any board and connected to any i2c bus.

At the same time we don't need to keep the as3645a_probe() after initializing
the device. Therefore we mark it and relevant functions with __devinit tag.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>