e1000e: workaround a Tx hang on 82577/82578

With bi-directional stress traffic, the receiver could hang causing the
hardware to stop and a "Detected Tx Unit Hang" message dumped to the system
logfile.  Temporarily workaround this issue by disabling Tx flow control by
default.  The issue is currently being investigated and a follow-on patch
will be provided to revert this when it is resolved.

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>