net: ll_temac: Avoid ndo_start_xmit returning NETDEV_TX_BUSY

[ Upstream commit f6396341194234e9b01cd7538bc2c6ac4501ab14 ]

As documented in Documentation/networking/driver.rst, the ndo_start_xmit
method must not return NETDEV_TX_BUSY under any normal circumstances, and
as recommended, we simply stop the tx queue in advance, when there is a
risk that the next xmit would cause a NETDEV_TX_BUSY return.

Signed-off-by: Esben Haabendal <esben@geanix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>