IB/qib: Allow PSM to select from multiple port assignment algorithms

We used to allow only full specification, or using all contexts within
an HCA before moving to the next HCA.  We now allow an additional
method -- round-robining through HCAs -- and make that the default.

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