staging: vc04_services: Fix NULL ptr sparse warnings

In calls to queue_message() in vchiq_core.c, the "void *context"
parameter is set as 0 rather than NULL. This patch amends each call to
use the proper NULL pointer. The following sparse warnings are fixed:

drivers/staging/vc04_services/interface/vchiq_arm/vchiq_core.c:1623:23: warning: Using plain integer as NULL pointer
drivers/staging/vc04_services/interface/vchiq_arm/vchiq_core.c:1976:47: warning: Using plain integer as NULL pointer
drivers/staging/vc04_services/interface/vchiq_arm/vchiq_core.c:2075:47: warning: Using plain integer as NULL pointer
drivers/staging/vc04_services/interface/vchiq_arm/vchiq_core.c:2095:47: warning: Using plain integer as NULL pointer
drivers/staging/vc04_services/interface/vchiq_arm/vchiq_core.c:2907:39: warning: Using plain integer as NULL pointer
drivers/staging/vc04_services/interface/vchiq_arm/vchiq_core.c:2929:39: warning: Using plain integer as NULL pointer
drivers/staging/vc04_services/interface/vchiq_arm/vchiq_core.c:3059:72: warning: Using plain integer as NULL pointer
drivers/staging/vc04_services/interface/vchiq_arm/vchiq_core.c:3860:31: warning: Using plain integer as NULL pointer
drivers/staging/vc04_services/interface/vchiq_arm/vchiq_core.c:3870:31: warning: Using plain integer as NULL pointer
drivers/staging/vc04_services/interface/vchiq_arm/vchiq_core.c:3880:31: warning: Using plain integer as NULL pointer

Signed-off-by: Mike Kofron <mpkofron@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>