can: flexcan: add ethtool support to get rx/tx ring parameters

This patch adds ethtool support to get the number of message buffers
configured for reception/transmission, which may also depends on
runtime configurations such as the 'rx-rtr' flag state.

Link: https://lore.kernel.org/all/20220108181633.420433-1-dario.binacchi@amarulasolutions.com


Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
[mkl: port to net-next/master, replace __sw_hweight64 by simpler calculation]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>