IB/ipath: Fix the mtrr_add args for chips with 2 buffer sizes

The values passed have never been right for iba 6120 chips, but just
happened to work.  We needed to select the right buffer offset in the
chip (both are in same register), and the total length was wrong also,
but was covered by the rounding up.

Signed-off-by: Dave Olson <dave.olson@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>