staging: brcm80211: remove macro definition DRV_MODULE_NAME

The macro definition DRV_MODULE_NAME has exactly the same value as
KBUILD_MODNAME so using that instead now.

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