IB/qib: Fix context allocation with multiple HCAs

The driver was incorrectly choosing HCAs on which to allocate new user
contexts based on overall count of usable ports regardless whether the
usable port was on the currently selected HCA.

Signed-off-by: Mike Marciniszyn <mike.marciniszyn@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>