MPP_F6281_MASK would be previously be returned when on mv88f6282,
which would disallow some valid MPP configurations.

Commit 830f8b91 (arm: plat-orion: fix printing of "MPP config
unavailable on this hardware") made this problem visible as an invalid
MPP configuration is now correctly detected and not applied.
Signed-off-by: Nicolas Schichan <nschichan@freebox.fr>
Cc: <stable@vger.kernel.org> # v3.9.x
Signed-off-by: Jason Cooper <jason@lakedaemon.net>