can: m_can: Batch FIFO writes during CAN transmit

Give FIFO writes the same treatment as reads to avoid fixed costs of
individual transfers on a slow bus (e.g., tcan4x5x).

Link: https://lore.kernel.org/r/20210817050853.14875-4-matt@bitbashing.io
Signed-off-by: Matt Kline <matt@bitbashing.io>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>