drivers: watchdog: wdt_nrfx.c: Fix error code value

Align driver implementation to the watchdog driver API.
https://docs.zephyrproject.org/latest/hardware/peripherals/watchdog.html



int wdt_disable(const struct device *dev)
shall return:
    0 – If successful.
    -EFAULT – If watchdog instance is not enabled.
    -EPERM – If watchdog can not be disabled directly by application code.
    -errno – In case of any other failure.

Signed-off-by: Sebastian Głąb <sebastian.glab@nordicsemi.no>