net/ethernet/freescale: rework quiesce/activate for ucc_geth

ugeth_quiesce/activate are used to halt the controller when there is a
link change that requires to reconfigure the mac.

The previous implementation called netif_device_detach(). This however
causes the initial activation of the netdevice to fail precisely because
it's detached. For details, see [1].

A possible workaround was the revert of commit
net: linkwatch: add check for netdevice being present to linkwatch_do_dev
However, the check introduced in the above commit is correct and shall be
kept.

The netif_device_detach() is thus replaced with
netif_tx_stop_all_queues() that prevents any tranmission. This allows to
perform mac config change required by the link change, without detaching
the corresponding netdevice and thus not preventing its initial
activation.

[1] https://lists.openwall.net/netdev/2020/01/08/201

Signed-off-by: Valentin Longchamp <valentin@longchamp.me>
Acked-by: Matteo Ghidoni <matteo.ghidoni@ch.abb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>