e1000e: flow control doesn't re-enable

When changing flow control (pause) parameters, the flow control thresholds
(i.e. when to send XON/XOFF frames) may not be setup correctly on parts
with copper media.  Call the existing e1000_set_fc_watermarks()
function to set these thresholds.

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>