IB/ipath: Fix spi_pioindex value

ipath_piobufbase was a single value offset, but is multiple values on
newer chips, so use only the 32 bits for the 2K buffers (4K buffers
are currently used only by the driver).

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