netdev: Make set() value parameter const void *

cpu/cc2538/radio/cc2538_rf_netdev.c

cpu/native/netdev_tap/netdev_tap.c

cpu/nrf5x_common/radio/nrfmin/nrfmin.c

drivers/at86rf2xx/at86rf2xx_netdev.c

drivers/cc110x/cc110x-netdev.c

drivers/cc2420/cc2420_getset.c

drivers/cc2420/cc2420_netdev.c

drivers/enc28j60/enc28j60.c

drivers/include/cc2420.h

drivers/include/net/netdev.h

drivers/include/net/netdev/eth.h

drivers/include/net/netdev/ieee802154.h

drivers/include/nrf24l01p.h

drivers/kw2xrf/kw2xrf_netdev.c

drivers/mrf24j40/mrf24j40_netdev.c

drivers/netdev_eth/netdev_eth.c

drivers/netdev_ieee802154/netdev_ieee802154.c

drivers/nrf24l01p/nrf24l01p.c

drivers/sx127x/sx127x_netdev.c

drivers/xbee/xbee.c