can: m_can: Batch acknowledge rx fifo

Instead of acknowledging every item of the fifo, only acknowledge the
last item read. This behavior is documented in the datasheet. The new
getindex will be the acknowledged item + 1.

Signed-off-by: Markus Schneider-Pargmann <msp@baylibre.com>
Link: https://lore.kernel.org/all/20221206115728.1056014-8-msp@baylibre.com
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>