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

This reverts commits 65f0b417,
d88d6b05,
fcfa0604,
747df225 and
867955f5.

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>