gpio: ep93xx: Fix port F hwirq numbers in handler

Fix wrong translation of irq numbers in port F handler, as ep93xx hwirqs
increased by 1, we should simply decrease them by 1 in translation.

Fixes: 482c2727 ("ARM: ep93xx: renumber interrupts")
Signed-off-by: Nikita Shubin <nikita.shubin@maquefel.me>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>