can: sja1000_isa: fix "limited range" compiler warnings

This patch fixes the compiler warnings: "comparison is always
false due to limited range of data type" by using "0xff" instead
of "-1" for unsigned values.

Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>
Acked-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>