staging: bcm2835-camera: Convert client_context field to a 32 bit handle

The client_context field is passed around which is really just a pointer
to a msg_context.  A lookup table mechanism for msg_context was added
previously, so convert this field to a handle as well.

The firmware never interperates the client_context, just passed it back.

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