net: hostname: fix logging hostname string

Following errors occuring after enabling debug logs:

  log: argument 2 in source net_hostname log message "%s: (%s): \
    Hostname set to %s" missinglog_strdup().
  log: argument 2 in source net_hostname log message "%s: (%s): \
    New hostname %s" missinglog_strdup().

Fix that by printing CONFIG_NET_HOSTNAME directly in the first case and
using log_strdup() in the second.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>