drivers: gpio_atmel: Fix erronous if statement

The GPIO_INT_ACTIVE_LOW value is  zero so the mask assignement is
never executed. Using the bit complement GPIO_INT_ACTIVE_HIGH the
proper mask is assigned

This issue was reported by Coverity

Coverity-CID: 151966

Change-Id: Ibc7d2e4c3ebee249b5ab9719f8177cc14c0d1d33
Signed-off-by: Sergio Rodriguez <sergio.sf.rodriguez@intel.com>