IXP: Fix GPIO_INT_ACT_LOW_SET()

The GPIO_INT_ACT_LOW_SET was incorrectly handling interrupt lines higher than 7.
This is due to the fact that there are two registers for total of 16 lines.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Bryan Hundven <bryanhundven@gmail.com>
Cc: Michael Schwingen <rincewind@discworld.dascon.de>