net: tftpsrv: Get correct client MAC address

NetServerEther was not being cleared in the tftp server code, so the
destination MAC address would be whatever the last destination MAC
address was.

Scenario:
U-Boot:
	dhcp
	tftpsrv
Host:
	Send device WRQ
Device:
	Responds with ACK to dhcp server mac address with
	host ip address

By clearing NetServerEther, we force a lookup of the host MAC address
to go with the associated host IP.
Signed-off-by: NAndrew Ruder <andrew.ruder@elecsyscorp.com>

net/tftp.c

@@ -849,6 +849,9 @@ TftpStartServer(void)
 
 	TftpState = STATE_RECV_WRQ;
 	net_set_udp_handler(TftpHandler);
+
+	/* zero out server ether in case the server ip has changed */
+	memset(NetServerEther, 0, 6);
 }
 #endif /* CONFIG_CMD_TFTPSRV */