[e1000] flow control updates

[e1000] flow control updates

* handle ethtool force flow control
* correctly set flow control hi/low watermarks based on size of Rx FIFO
  area.  The size can change if doing Jumbo Frames or, in the case of
  82547, is smaller to start with.
* was not properly forcing flow control settings to fc_none if using
  strict IEEE flow control override.