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>