can: usb_8dev: 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: 0024d8ad

 ("can: usb_8dev: Add support for USB2CAN interface from 8 devices")
Link: https://lore.kernel.org/all/20220719143550.3681-10-mailhol.vincent@wanadoo.fr
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>