staging: brcm80211: remove volatile keyword from driver sources

As checkpatch.pl rightfully indicated the volatile keyword was used
but not necessary. It has been removed from the code.

Signed-off-by: Arend van Spriel <arend@broadcom.com>
Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>