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Since commit 2f1e8ea7 ("net: dsa: link interfaces with the DSA
master to get rid of lockdep warnings"), DSA gained a requirement which
it did not fulfill, which is to unlink itself from the DSA master at
shutdown time.

Since the Arrow SpeedChips XRS700x driver was introduced after the bad
commit, it has never worked with DSA masters which decide to unregister
their net_device on shutdown, effectively hanging the reboot process.
To fix that, we need to call dsa_switch_shutdown.

These devices can be connected by I2C or by MDIO, and if I search for
I2C or MDIO bus drivers that implement their ->shutdown by redirecting
it to ->remove I don't see any, however this does not mean it would not
be possible. To be compatible with that pattern, it is necessary to
implement an "if this then not that" scheme, to avoid ->remove and
->shutdown from being called both for the same struct device.

Fixes: ee00b24f ("net: dsa: add Arrow Spee...