mvneta: drop redundant mac address check

Checking if MAC address is valid using is_valid_ether_addr() is already done in
of_get_mac_address().

Signed-off-by: Luka Perkov <luka@openwrt.org>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
CC: David Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>