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>

SVN-Revision: 45937