e1000e: partial revert of 3ec2a2b8 plus FC workraround for 82577/8

Commit 3ec2a2b8 broke Tx/Rx when using
jumbo frames on certain parts (i.e. only PAUSE frames could be exchanged
once the high water mark was reached preventing normal packet traffic).
This patch reverts the breakage and sets appropriate high and low water
marks of the Rx FIFO for 82577/82578 which require a workaround due to a
flow control issue in hardware.

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>