Extend the coalescing implementation for transmits.

In normal mode the chip raises an interrupt for every finished transmit.
This implementation switches to coalescing mode as soon as an interrupt
handled a transmit. For coalescing the watermark level interrupt is used
to interrupt exactly after x frames were sent. It switches back into
normal mode once there was an interrupt with no finished transmit and
the timer being inactive.

The timer is shared with receive coalescing. The time for receive and
transmit coalescing timers have to be the same for that to work. The
benefit is to have only a single running timer.
Signed-off-by: Markus Schneider-Pargmann <msp@baylibre.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/all/20240207093220.2681425-6-msp@baylibre.comSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>