net: tftpsrv: Get correct client MAC address

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: Andrew Ruder <andrew.ruder@elecsyscorp.com>