staging: vc04_services: Add casts to remove warnings in vchiq_core.c

When compiling vchiq_core.c for 64 bit, the compiler
emits a few warnings that are not actual issues. This
change adds a few casts to remove the extra unnecessary
warnings.

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