can: mcp251xfd: add TX IRQ coalescing ethtool support

This patch adds support ethtool based configuration for the TX IRQ
coalescing added in the previous patch.

Link: https://lore.kernel.org/20220313083640.501791-12-mkl@pengutronix.de


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