MIPS: BCM47XX: Fix button inversion for Asus WL-500W

The Asus WL-500W buttons are active high, but the software treats them
as active low. Fix the inverted logic.

Fixes: 3be97255 ("MIPS: BCM47XX: Import buttons database from OpenWrt")
Signed-off-by: Mirko Parthey <mirko.parthey@web.de>
Acked-by: Rafał Miłecki <rafal@milecki.pl>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: linux-mips@linux-mips.org
Cc: <stable@vger.kernel.org> # 3.14.x-
Patchwork: https://patchwork.linux-mips.org/patch/15295/


Signed-off-by: James Hogan <james.hogan@imgtec.com>