ixgbe: cache AUTOC reads

This patch removes majority of the AUTOC register reads by using a cached
value instead.

The reason for this change is to avoid writing corrupted values to AUTOC
due to bad FW.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>