IB/mthca: Don't use privileged UAR for kernel access

Make kernel use UAR2 instead of UAR1 for hardware access: this adds
sanity checking from the hardware side, without any performance cost.

Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>