can: m_can: Implement transmit submission coalescing

can: m_can: Implement transmit submission coalescing

m_can supports submitting multiple transmits with one register write.
This is an interesting option to reduce the number of SPI transfers for
peripheral chips.

The m_can_tx_op is extended with a bool that signals if it is the last
transmission and the submit should be executed immediately.

The worker then writes the skb to the FIFO and submits it only if the
submit bool is set. If it isn't set, the worker will write the next skb
which is waiting in the workqueue to the FIFO, etc.

Signed-off-by: Markus Schneider-Pargmann <msp@baylibre.com>
Link: https://lore.kernel.org/all/20240207093220.2681425-15-msp@baylibre.com
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>