kernel: timer: Fix incorrect behavior for timers with K_FOREVER period

Zephyr docs state that timers will act as one-shot timers when started
with a period of K_NO_WAIT or K_FOREVER. However the code adjusting
period was setting K_FOREVER timeout ticks to 1 which caused the timer
to expire every tick. This adds a check to not adjust K_FOREVER periods

Signed-off-by: Eric Johnson <eric@liveathos.com>