time_t NTPClient::get_timestamp() incorrect error logic

The problem:
time_t is defined as:

typedef unsigned int time_t; /* date/time in unix secs past 1-Jan-70 */

therefore, get_timestamp() can't return a negative value on error.
Instead, it should return 0 on error or be changed as follows to be consistent with error report in other Mbed OS modules:
```

int NTPClient::get_timestamp(time_t *time, int timeout)
{
const time_t TIME1970 = (time_t)2208988800UL;
int ntp_send_values[12] = {0};
int ntp_recv_values[12] = {0};

SocketAddress nist;

if (iface)
{
int ret_gethostbyname = iface->gethostbyname(nist_server_address, &nist);

if (ret_gethostbyname < 0) {
// Network error on DNS lookup
return ret_gethostbyname;
}

nist.set_port(nist_server_port);

memset(ntp_send_values, 0x00, sizeof(ntp_send_values));
ntp_send_values[0] = '\x1b';

memset(ntp_recv_values, 0x00, sizeof(ntp_recv_values));

UDPSocket sock;
sock.open(iface);
sock.set_timeout(timeout);

sock.sendto(nist, (void*)ntp_send_values, sizeof(ntp_send_values));

SocketAddress source;
const int n = sock.recvfrom(&source, (void*)ntp_recv_values, sizeof(ntp_recv_values));

if (n > 10) /* success */
{
*time = ntohl(ntp_recv_values[10]) - TIME1970;

return NSAPI_ERROR_OK; /* return 0 if no error */

} else {
if (n < 0)
{
// Network error
return n;

} else {
// No or partial data returned
return -1;
}
}

} else {
// No network interface
return -2;
}
}

```

Start at the NTPClient::get_timestamp() entry point and inspect its callers to understand the expected error convention. Reproduce the failure around the unsigned time_t return behavior, then verify that the chosen error handling is consistent with callers and existing Mbed OS module conventions.

Written by the indexing model from the issue text.