can: sja1000: add read/write routines for 8, 16 and 32-bit register access

add routines for 8, 16 and 32-bit access like in
drivers/i2c/busses/i2c-pca-platform.c

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Acked-by: Wolfgang Grandegger <wg@grandegger.com>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>