can: slcan: add ethtool support to reset adapter errors

This patch adds a private flag to the slcan driver to switch the
"err-rst-on-open" setting on and off.

"err-rst-on-open" on  - Reset error states on opening command

"err-rst-on-open" off - Don't reset error states on opening command
                        (default)

The setting can only be changed if the interface is down:

    ip link set dev can0 down
    ethtool --set-priv-flags can0 err-rst-on-open {off|on}
    ip link set dev can0 up

Link: https://lore.kernel.org/all/20220628163137.413025-11-dario.binacchi@amarulasolutions.com
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>