include/netinet/arp.h: change the type of arp_dev from uint8_t to char

the same variable type in linux is char, in order to avoid modifying
the third-party library code when porting the third-party library code.

Signed-off-by: zhanghongyu <zhanghongyu@xiaomi.com>

net/netdev/netdev_ioctl.c

@@ -1361,7 +1361,7 @@ static bool ioctl_arpreq_parse(FAR struct arpreq *req,
     {
       *addr = (FAR struct sockaddr_in *)&req->arp_pa;
       *dev  = req->arp_dev[0] != '\0' ?
-              netdev_findbyname((FAR const char *)req->arp_dev) :
+              netdev_findbyname(req->arp_dev) :
               netdev_findby_ripv4addr(INADDR_ANY, (*addr)->sin_addr.s_addr);
       return true;
     }