net: lib: sntp_simple: Handle responses from previous iterations

SNTP simple runs request iterations with exponential backoff.
If the net interface is a slower connection (ie. CAT M1 modems)
then the request will be sent but the response may take time to
be received, thus causing a timeout and another request to be sent.
Because of the nature of UDP and the fact that the same socket
(source IP/port combo) is being used for both requests, a delayed
response to the first request can be received as the response to the
second request, causing -EINVAL to be returned when the timestamps
mismatch (see subsys/net/lib/sntp/sntp.c). The solution provided
retries receiving the response when the timestamp is mismatched
(without sending an additional request).

Signed-off-by: Marcus Penate <marcus.penate@ellenbytech.com>