IB/mthca: Relax UAR size check

There are some cards around that have UAR (user access region) size
different from 8 MB.  Relax our sanity check to make sure that the PCI
BAR is big enough to access the UAR size reported by the device
firmware instead.

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