ice: Rename low_power_ctrl

Rename the low_power_ctrl field to low_power_ctrl_an to be properly
descriptive of it being an autoneg field.

Signed-off-by: Lev Faerman <lev.faerman@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>