Update gpio_irq_api.c

enable 16 irq channels, expanding EXTI9_5_IRQn and EXTI15_10_IRQn
Revert "Merge pull request #265 from cfb95/patch-1"

This reverts commit 6b9fca177ab428a93a520d666ca5b4aeb921da51, reversing
changes made to c9167e8c43e88993cf02fe17587f2206fa1bba2e.
Merge branch 'patch-3' of git://github.com/cfb95/mbed into cfb95-patch-3