drivers: counter: counter_ambiq_timer: Enable interrupt in set_alarm

Alarm interrupt is disabled in cancel_alarm, we should re-enable it
in set_alarm, at meanwhile, should reset the compare register in
cancel_alarm to avoid the contention condition in
cancel_alarm & set_alarm in short time.
This change fixes the test case failure at
zephyr\tests\drivers\counter\counter_basic_api.

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