IB/ipath: fix handling of kpiobufs

Change comment: no longer imply that user can set ipath_kpiobufs to zero.
Actually set ipath_kpiobufs from parameter. Previously only altered
per-device ipath_lastport_piobuf, which was over-written in chip init.

Signed-off-by: Bryan O'Sullivan <bryan.osullivan@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>