kernel/timer: Handle K_FOREVER in k_timer_start()

The possibility of passing K_FOREVER as the initial duration argument
to k_timer_start() wasn't being handled, with the result that the
computed value became an zero timeout (effecitvely treating it as
K_NO_WAIT and firing at the next tick).

This is obviously pathlogical, but it should still do what the code
says it should and wait forever.

Make k_timer_start(..., K_FOREVER, ...) a noop.

Fixes #25820

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>