[ARM] Fix iop3xx macro

Macro arguments used in expressions should have parens.  This avoids
warnings such as:

drivers/serial/8250.c:1951: warning: suggest parentheses around arithmetic in operand of |

Acked-by: Dan Williams <dan.j.williams@intel.com>
Acked-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>