ixgbe: Fix RSC completion delay causing Rx interrupts to stop

When a user disables interrupt throttling with ethtool on 82599 devices,
the interrupt timer may not be re-enabled if hardware RSC is running.  The
RSC completions in hardware don't complete before the next ITR event tries
to fire, so the ITR timer never gets re-armed.  This patch increases the
amount of time between interrupts when throttling is disabled (rx-usecs =
0) when the hardware RSC deature is enabled.

Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>