can: m_can: Count read getindex in the driver

The getindex gets increased by one every time. We can calculate the
correct getindex in the driver and avoid the additional reads of rxfs.

Signed-off-by: Markus Schneider-Pargmann <msp@baylibre.com>
Link: https://lore.kernel.org/all/20221206115728.1056014-6-msp@baylibre.com


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>