can: sun4i_can: 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: 0738eff1

 ("can: Allwinner A10/A20 CAN Controller support - Kernel module")
Link: https://lore.kernel.org/all/20220719143550.3681-7-mailhol.vincent@wanadoo.fr
CC: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>