IB/qib: Don't mark VL15 bufs as WC to avoid a rare 7322 chip problem

Don't set write combining via PAT on the VL15 buffers to avoid a rare
problem with unaligned writes from interrupt-flushed store buffers.

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