net/can/mscan: add __iomem annotations

This patch fixes the following sparse warning by adding the missing
__iomem annotation.

drivers/net/can/mscan/mscan.c:73:32: warning: incorrect type in argument 1 (different address spaces)
drivers/net/can/mscan/mscan.c:73:32:    expected unsigned char volatile [noderef] [usertype] <asn:2>*addr
drivers/net/can/mscan/mscan.c:73:32:    got unsigned char *<noident>

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>