arc_emac: drop redundant mac address check

Checking if MAC address is valid using is_valid_ether_addr() is already done in
of_get_mac_address(). While at it, reorganize checking so it matches checks in
other drivers.

Signed-off-by: Luka Perkov <luka@openwrt.org>
CC: Alexey Brodkin <Alexey.Brodkin@synopsys.com>
CC: David Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>