can: c_can: add ethtool support

With commit 132f2d45 ("can: c_can: add support to 64 message
objects") the number of message objects used for reception /
transmission depends on FIFO size.

The ethtools API support allows you to retrieve this info. Driver info
has been added too.

Link: https://lore.kernel.org/r/20210514165549.14365-2-dariobin@libero.it


Signed-off-by: Dario Binacchi <dariobin@libero.it>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>