can: m_can: Batch FIFO reads during CAN receive

On peripherals communicating over a relatively slow SPI line
(e.g. tcan4x5x), individual transfers have high fixed costs.
This causes the driver to spend most of its time waiting between
transfers and severely limits throughput.

Reduce these overheads by reading more than one word at a time.
Writing could get a similar treatment in follow-on commits.

Link: https://lore.kernel.org/r/20210817050853.14875-3-matt@bitbashing.io


Signed-off-by: Matt Kline <matt@bitbashing.io>
[mkl: remove __packed from struct id_and_dlc]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>