e1000e: correct flow control thresholds

The flow control thresholds, i.e. high and low watermarks of the Rx
FIFO for when the hardware should transmit PAUSE frames (XON and XOFF,
respectively), need to be tuned for more efficient use of the FIFO.
The logic to set the thresholds for parts that support early-receive
(ERT) was also wrong in that it should check whether jumbo frames are
in use.

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>