can: kvaser_usb_hydra: do not report txerr and rxerr during bus-off

During bus off, the error count is greater than 255 and can not fit in
a u8.

Fixes: aec5fb22

 ("can: kvaser_usb: Add support for Kvaser USB hydra family")
Link: https://lore.kernel.org/all/20220719143550.3681-8-mailhol.vincent@wanadoo.fr
CC: Jimmy Assarsson <extja@kvaser.com>
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>