ixgbe: Add register wait for slow links

Use a new register to wait for previous register writes to complete
before issuing a register read. This is needed when slower links
are in use.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>