staging: vc04_services: fix up rpi firmware functions

The raspberrypi-firmware.h file should provide empty functions if we
aren't building in that option.  This makes it easier to test-build
code, and not have odd warnings about unused variables if you just try
to #define away the functions.

Cc: Daniel Stone <daniels@collabora.com>
Cc: "Noralf Trønnes" <noralf@tronnes.org>
Cc: Pranith Kumar <bobby.prani@gmail.com>
Cc: popcornmix <popcornmix@gmail.com>
Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>