e1000e: sync change flow control variables with ixgbe

Sync flow control variables and usage model with that found in the ixgbe
driver.

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>