Because we probe the master ourselves (and fail if there is no master),
it is not possible that we don't have a master device.

There is one catch though: device removal. We don't support that. It
wasn't supported neither before this patch. Because the master device
was only set in .pre_probe(), if a device was removed master_dev was a
dangling pointer and transmitting a frame cause a panic. I don't see a
good solution without having some sort of notify machanism when a
udevice is removed.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Michael Walle <michael@walle.cc> [DSA unit tests]
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>