ARM: orion5x: fixup 5181 MPP mask check

During the refactoring of the Orion MPP code, the detection for
the 5181l as been used to select the 5181 MPP mask, which is wrong.
Select the 5181 mask for all 5181 variants.

Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>