net: openthread: Fix alarm timers reference calculation

The OpenThread stack uses uint32_t to calculate expiry time for
alarms, while comparing to zephyr's uint64_t uptime.

This commit fixes broken milliseconds alarms after ~49.7 days of
uptime.

Fixes #39704

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>