b53: widen stp state mask to 3 bits (instead of 2)

b53: widen stp state mask to 3 bits (instead of 2)

At least on my b53 chip, the mask is 3 bits wide, and because
of this some STP states are not set properly and discarded when read.

Maybe for some other chips it makes sense to have just 2 bits width,
but I don't have other versions around to test/validate.

If that's the case then maybe we could add another STP state mask.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Signed-off-by: Jonas Gorski <jogo@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45937 3c298f89-4303-0410-b956-a3cf2f4a3e73