e1000e: potentially incorrect return for e1000_set_d3_lplu_state_ich8lan

In the unlikely event that e1e_wphy() returns an error, the returned error
code is not propogated to the caller of e1000_set_d3_lplu_state_ich8lan().
With this change, there is a rare possibility that ret_val might not get
set so it must be initialized.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>