ixgbe: Limit SFP polling rate

Reduce the frequency of polling for SFP modules. Because the
service task sometimes runs at high rates, we can poll for
SFPs too often. When an SFP is not present, the I2C timeouts
that result are very costly. So, prevent SFP polling from
being done more than once every two seconds. To reduce latency,
the poll time is cleared in a couple of cases to permit the
next service task execution to poll the SFP module.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>