drivers: udc_ambiq: defer enqueue event if the endpoint is stalled

Ambiq USB HAL do not expect endpoint transaction request when an
endpoint is stalled. This commit addresses this behavior in shim driver
by checking for endpoint's stall status when enqueue request is
received, and defer it until endpoint stall is cleared.

Signed-off-by: Chew Zeh Yang <zeon.chew@ambiq.com>