esp32/network_lan: Make power arg to constructor optional.

A value of None for this argument is already supported, so the argument can
be made optional.

ports/esp32/network_lan.c

@@ -99,7 +99,7 @@ STATIC mp_obj_t get_lan(size_t n_args, const mp_obj_t *pos_args, mp_map_t *kw_ar
         { MP_QSTR_id,           MP_ARG_OBJ, {.u_obj = mp_const_none} },
         { MP_QSTR_mdc,          MP_ARG_KW_ONLY | MP_ARG_REQUIRED | MP_ARG_OBJ },
         { MP_QSTR_mdio,         MP_ARG_KW_ONLY | MP_ARG_REQUIRED | MP_ARG_OBJ },
-        { MP_QSTR_power,        MP_ARG_KW_ONLY | MP_ARG_REQUIRED | MP_ARG_OBJ },
+        { MP_QSTR_power,        MP_ARG_KW_ONLY | MP_ARG_OBJ, {.u_obj = mp_const_none} },
         { MP_QSTR_phy_addr,     MP_ARG_KW_ONLY | MP_ARG_REQUIRED | MP_ARG_INT },
         { MP_QSTR_phy_type,     MP_ARG_KW_ONLY | MP_ARG_REQUIRED | MP_ARG_INT },
         { MP_QSTR_clock_mode,   MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = -1} },