e1000e: workaround DMA unit hang on I218

At 1000Mbps link speed, one of the MAC's internal clocks can be stopped for
up to 4us when entering K1 (a power mode of the MAC-PHY interconnect).  If
the MAC is waiting for completion indications for 2 DMA write requests into
Host memory (e.g. descriptor writeback or Rx packet writing) and the
indications occur while the clock is stopped, both indications will be
missed by the MAC causing the MAC to wait for the completion indications
and be unable to generate further DMA write requests.  This results in an
apparent hardware hang.

Work-around the issue by disabling the de-assertion of the clock request
when 1000Mbps link is acquired (K1 must be disabled while doing this).

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>