staging: brcm80211: remove unused definition flag from top Makefile

The use of the definition BCMDMA32 was already removed from the
source code, but it was still defined in the top-level Makefile.
This patch removes it.

Reported-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>