staging/bcm2835-camera: Set ourselves up as a platform driver.

This allows bcm2835-camera to automatically probe after VCHI has
loaded, rather than only successfully probing if the arbitrary probe
order chooses us after VCHI.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>