Revert "sfc: Use write-combining to reduce TX latency" and follow-ups

Revert "sfc: Use write-combining to reduce TX latency" and follow-ups

This reverts commits d775b3584483d512dc98407f91f58f008e80e718,
9889d7d4e70e90b08b2924aa8e8dc999583ad33d,
34d977a58cf770166682876c0c775e738fc2f947,
d80866d93d48893a13152e0c1313f75ea06fcc17 and
5b971cdc07a41a437ce6b50fc1b0e4e178fdebdd.

Depending on the processor model, write-combining may result in
reordering that the NIC will not tolerate.  This typically results
in a DMA error event and reset by the driver, logged as:

sfc 0000:0e:00.0: eth2: TX DMA Q reports TX_EV_PKT_ERR.
sfc 0000:0e:00.0: eth2: resetting (ALL)

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>