can: kvaser_usb_leaf: 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: 7259124e

 ("can: kvaser_usb: Split driver into kvaser_usb_core.c and kvaser_usb_leaf.c")
Link: https://lore.kernel.org/all/20220719143550.3681-9-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>