igb: check for packets on all tx rings when link is down

We were previously only checking the first tx ring to see if it had any
packets in it when the link when down.  However we should be checking all
of the rings so this patch makes it so that all of the rings are now being
checked.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>