gnrc_netdev2: harmonize naming scheme to rest of GNRC

drivers/cc110x/gnrc_cc110x/gnrc_netdev2_cc110x.c

drivers/cc110x/include/gnrc_netdev2_cc110x.h

sys/auto_init/netif/auto_init_cc110x.c

sys/auto_init/netif/auto_init_encx24j600.c

sys/auto_init/netif/auto_init_netdev2_tap.c

sys/include/net/gnrc/netdev2.h

sys/include/net/gnrc/netdev2/eth.h

sys/net/gnrc/link_layer/netdev2/gnrc_netdev2.c

sys/net/gnrc/link_layer/netdev2/gnrc_netdev2_eth.c