can: m_can: Count TXE FIFO getidx in the driver

The getindex simply increases by one for every iteration. There is no
need to get the current getidx every time from a register. Instead we
can just count and wrap if necessary.

Signed-off-by: Markus Schneider-Pargmann <msp@baylibre.com>
Link: https://lore.kernel.org/all/20221206115728.1056014-5-msp@baylibre.com


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