net: rename and refactor eth_rand_ethaddr() function

Some functions in include/net.h are ported from
include/linux/etherdevice.h of Linux Kernel.

For ex.
  is_zero_ether_addr()
  is_multicast_ether_addr()
  is_broadcast_ether_addr()
  is_valid_ether_addr();

So, we should use the same function name as that of Linux Kernel,
eth_rand_addr(), for consistency.

Besides, eth_rand_addr() has been implemented as an inline function.
So it should not be surrounded by #ifdef CONFIG_RANDOM_MACADDR.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>