net: sntp: get rid of the callback function

The original SNTP client library was designed for the net-app API, for
which it makes sense to have a callback function, which is called
asynchronously when an answer is received.

For the socket based interface, the callback is called just before
sntp_request() returns. It gets the status and the epoch_time in
parameter, however the status is already returned by sntp_request(). It
therefore make sense to replace the callback function by a pointer to
epoch_time.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>