e1000e: flow control thresholds not correct when changing mtu

When changing MTU, save it off prior to resetting otherwise the flow control
thresholds may be miscalculated.

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>