e1000: Neaten e1000_config_dsp_after_link_change

Separate a complicated bit of e1000_config_dsp_after_link_change
into a new static function e1000_1000Mb_check_cable_length.

Reduces indentation and adds a bit of clarity.

Signed-off-by: Joe Perches <joe@perches.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>