IB/mthca: Make fw_cmd_doorbell default to 0

Setting fw_cmd_doorbell allows FW command to be queued using posted
writes instead of requiring polling on a "go" bit, so it should be a
performance boost.  However, the option causes problems with at least
some device/firmware combinations, so set the default to 0 until we
understand what's going on better.

Signed-off-by: Roland Dreier <rolandd@cisco.com>