IB/qib: Display correct value for number of contexts

A "fix" for a bug with the number of contexts on a single-port board
caused the calculation to be off by one, which causes problems with
the upper layers.  The same problem exists for number of free
contexts, which is also fixed here.

Signed-off-by: Mitko Haralanov <mitko.haralanov@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>