i2c: ismt: mark register space with __iomem

This fixes the code to suppress sparse warnings like:

drivers/i2c/busses/i2c-ismt.c:725:36: warning: incorrect type in argument 2 (different address spaces)
drivers/i2c/busses/i2c-ismt.c:725:36:    expected void volatile [noderef] <asn:2>*addr
drivers/i2c/busses/i2c-ismt.c:725:36:    got void *

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>