staging: bcm2835_camera: Convert control_service field of mmal_msg_header to u32

The camera driver passes messages back and forth between the firmware with
requests and replies.  One of the fields of the message header called
control_service is a pointer so the size changes between 32 bit and 64 bit.

Luckly, the field is not interperated by the driver, so it can be changed
to a u32 which has a fixed size.

Signed-off-by: Michael Zoran <mzoran@crowfest.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>