net: systemport: use standard netdevice notifier to detect DSA presence

The SYSTEMPORT driver maps each port of the embedded Broadcom DSA switch
port to a certain queue of the master Ethernet controller. For that it
currently uses a dedicated notifier infrastructure which was added in
commit 60724d4b ("net: dsa: Add support for DSA specific notifiers").

However, since commit 2f1e8ea7

 ("net: dsa: link interfaces with the
DSA master to get rid of lockdep warnings"), DSA is actually an upper of
the Broadcom SYSTEMPORT as far as the netdevice adjacency lists are
concerned. So naturally, the plain NETDEV_CHANGEUPPER net device notifiers
are emitted. It looks like there is enough API exposed by DSA to the
outside world already to make the call_dsa_notifiers API redundant. So
let's convert its only user to plain netdev notifiers.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>