task_wdt: fix overflow in current_ticks making wdt get stuck

The task_wdt was getting stuck after approx. 36 hours on e.g. nRF52840,
which has a SysTick with 32768 Hz. This corresponds to an overflow of
the uint32_t current_ticks in schedule_next_timeout.

This commit fixes the accidentally introduced narrowing conversion.

Fixes #40152

Signed-off-by: Martin Jäger <martin@libre.solar>