e1000e: set flow control thresholds properly after enabling/disabling pause

When flow control (pause) parameters were changed via ethtool (i.e. enabled
or disabled), the newly calculated thresholds were not being written to the
device for non-fiber media.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>