net/eth.c: eth_mac_skip() is only needed when CONFIG_NET_MULTI is set

Move it inside the #ifdef CONFIG_NET_MULTI to avoid

	eth.c:64: warning: 'eth_mac_skip' defined but not used

messages from anumber of old, non-CONFIG_NET_MULTI boards.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Ben Warren <biggerbadderben@gmail.com>