rpl-mesh-qemu sample, the net inface init failed.

[walter-xie]

[net/rpl] [DBG] dio_timer: (0x20000df0): Sending DIO later because IPv6 link local address is not found

CONFIG_NET_APP_SETTINGS=y
samples: rpl-mesh-qemu
_net_app_ieee802154_setup() function would return -EINVAL in line 51.
upipe_set_channel() and upipe_set_txpower() function should just "return 0;"

[nashif]

@rveerama1 ping

[rveerama1]

I will check.