s390/qeth: remove dead code in qeth_l3_iqd_read_initial_mac()

card->info.unique_id is always 0 for IQD devices, so don't bother with
copying it into the 0-initialized cmd.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>