timer: ambiq_stimer: fixing disabling tickless not working issue

In init function, start timer with period CYC_PER_TICK if tickless is
not enabled, This change is for fixing the issue that disabling
CONFIG_TICKLESS_KERNEL the OS tick is not work issue, this
causes the OS not starting scheduling correctly.

Signed-off-by: Bryan Zhu <bzhu@ambiq.com>