staging: fsl-dpaa2: ethsw: Add missing netdevice check

port_switchdev_event() does not check that the target network device is
actually backed by the ethsw driver, this could be problematic in a
stacked environment case.

Fixes: 44baaa43 ("staging: fsl-dpaa2/ethsw: Add Freescale DPAA2 Ethernet Switch driver")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>