IB/mthca: Fix thinko in init_mr_table()

Commit c20e20ab ("IB/mthca: Merge MR and FMR space on 64-bit systems")
swapped the number of MTTs and MPTs when initializing the MR table. As
a result, we get a kernel oops when the number of MTT segments
allocated exceeds 0x20000.

Noted by Troy Benjegerdes <troy@scl.ameslab.gov>, and reproduced by
Dotan Barak <dotanb@mellanox.co.il>.  This fixes
https://bugs.openfabrics.org/show_bug.cgi?id=490



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