staging: bcm2835-audio: bcm2835.h: fix macro coding style issue

Fix checkpatch.pl warning of the form "CHECK: Macro argument 'vol' may be
better as '(vol)' to avoid precedence issues."

Signed-off-by: Nathan Howard <adanhawthorn@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>