The network queue is currently always stopped in start_xmit and
continued in the interrupt handler. This is not possible anymore if we
want to keep multiple transmits in flight in parallel.

Use the previously introduced tx_fifo_in_flight counter to control the
network queue instead. This has the benefit of not needing to ask the
hardware about fifo status.

This patch stops the network queue in start_xmit if the number of
transmits in flight reaches the size of the fifo and wakes up the queue
from the interrupt handler once the transmits in flight drops below the
fifo size. This means any skbs over the limit will be rejected
immediately in start_xmit (it shouldn't be possible at all to reach that
state anyways).

The maximum number of transmits in flight is the size of the fifo.
Signed-off-by: Markus Schneider-Pargmann <msp@baylibre.com>
Link: https://lore.kernel.org/all/20240207093220.2681425-13-msp@baylibre.comSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>