drivers: watchdog: wdt_nrfx.c: Unistall timeouts in wdt_disable()

According to API description, wdt_disable() shall
automatically uninstall all timeouts.
https://docs.zephyrproject.org/latest/hardware/peripherals/watchdog.html



Implement missing wdt_disable() functionality:
- disable WDT RR channels;
- remove callback from timeout channels;
- set data->m_allocated_channels to zero.

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