igb: always use adapter->itr as EITR value

The igb driver was switching between adapter->itr containing the EITR value
and the number of interrupts per second.  This resulted in high latencies
being seen after brining the interface down and then back up.  To resolve
the issue the itr value will now only contain the value that should be
programmed into EITR.

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>