The else-if branch and else branch set mac_ok to true similarly,
so combine the two into single else branch.

Also add comments to explain the two conditions, which
from Michael Chan and Vasundhara Volam.
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Acked-by: NMichael Chan <michael.chan@broadcom.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>