cxgbe(4): Disable write-combined doorbells by default.

This had been the default behavior but was changed accidentally as part
of the recent iw_cxgbe+OFED overhaul. Fix another bug in that change
while here: the global knob affects all the adapters in the system and
should be left alone by per-adapter code.

MFC after: 3 days
Sponsored by: Chelsio Communications