eth: sp7021: fix use after free bug in spl2sw_nvmem_get_mac_address

This frees "mac" and tries to display its address as part of the error
message on the next line.  Swap the order.

Fixes: fd3040b9 ("net: ethernet: Add driver for Sunplus SP7021")
Signed-off-by: Zheng Wang <zyytlz.wz@163.com>
Signed-off-by: David S. Miller <davem@davemloft.net>