net: systemport: Check DSA notifier master against ourself

Check that the master network device that is signaled through the DSA
notifier is actually going to be ourself, otherwise, we could just be
de-referencing garbage from other drivers.

Fixes: 84ff33eeb23d ("net: systemport: Establish DSA network device queue mapping")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>