Fix below build warning if LWIP_6LOWPAN_IPHC=0.
../../../../lwip/src/netif/lowpan6.c:186:1: error: ‘lowpan6_get_address_mode_mc’ defined but not used [-Werror=unused-function]
lowpan6_get_address_mode_mc(const ip6_addr_t *ip6addr)
^~~~~~~~~~~~~~~~~~~~~~~~~~~
../../../../lwip/src/netif/lowpan6.c:160:1: error: ‘lowpan6_get_address_mode’ defined but not used [-Werror=unused-function]
lowpan6_get_address_mode(const ip6_addr_t *ip6addr, const struct ieee_802154_addr *mac_addr)
^~~~~~~~~~~~~~~~~~~~~~~~
Also correct comment typo: s/LWIP_6LOWPAN_HC/LWIP_6LOWPAN_IPHC/g
Signed-off-by: Axel Lin <axel.lin@ingics.com>