e1000e: 82571 Tx Data Corruption during Tx hang recovery

A bus trace shows that while executing e1000e_down, TCTL is cleared except
for the PSP bit.  This occurs while in the middle of fetching a TSO packet
since the Tx packet buffer is full at that point. Before the device is
reset, the e1000_watchdog_task starts to run from the middle (it was
apparently pre-empted earlier, although that is not in the trace) and sets
TCTL.EN.  At that point, 82571 transmits the corrupted packet, apparently
because TCTL.MULR was cleared in the middle of fetching a packet, which is
forbidden.

Driver should just clear TCTL.EN in e1000_reset_hw_82571 instead of
clearing the entire register, so as not to change any settings in the
middle of fetching a packet.

Signed-off-by: Tushar Dave <tushar.n.dave@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>