net: lwip: dhcp: make NTP servers usable by the sntp command

When both CMD_DHCP and CMD_SNTP are enabled, one would expect the NTP
servers received by DHCP to be used by the sntp command by default. Fix
dhcp_loop() so that it is indeed the case.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reported-by: Michal Simek <michal.simek@amd.com>