gpio: ep93xx: Properly call the chained IRQ handler

The chained irq handler should call chained_irq_enter()
and chained_irq_exit() before/after handling the chained
IRQ.

Acked-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>