drivers: pinctrl: silabs: Fix pin deallocation from digital bus

Fix the scenario where a pinctrl node intends to deallocate a pin
from a peripheral. If the GPIO mode is disabled the DBUS route
should be cleared, not set. This allows reuse of a pin for other
purposes when a driver is suspended and the pinctrl sleep state is
applied, as GPIOs are typically disabled in the sleep state.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>