drivers: watchdog: gecko: Fix API corner cases

Properly return the correct error codes when the watchdog API
is misused. Fail timeout install if WDT_FLAG_RESET_CPU_CORE is
set, a watchdog reset will at a minimum cause a soft reset of
the entire SoC.

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