drivers: esp_at: fix net_if_get_by_iface() typo

s/net_if_get_by_ifindex/net_if_get_by_iface/

Fixes: 0e57844b ("drivers: wifi: esp_at: Bind DNS to device net
  interface")
Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>