drivers: gpio_nrfx: Fix handling of GPIO_DISCONNECTED

The driver returns -EIO when a pin to be disconnected was not earlier
configured as input or output, what is not in line with the GPIO API.
This commit changes the driver to return 0 in such case.
Also -EIO is incorrectly returned when an interrupt trigger cannot be
configured in the nrfx_gpiote driver. This commit corrects this value
to -EINVAL.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>