gpio: stm32: support disabling and reenabling interrupts on pin

Up to now interrupts could be only configured once, with no way to
disable them in runtime.

Allow interrupts to be disabled in runtime and then properly reenabled
on user request. This allows to ignore interrupts when software is not
expecting them.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>