net: dns: Return error code

Return ret error code instead of always returning zero causing
warnings:

...
subsys/net/lib/dns/resolve.c:975:6: warning: variable 'ret' set but
not used [-Wunused-but-set-variable]
        int ret = 0;
            ^
...

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>