IB/qib: Correct sense on freectxts increment and decrement

Commit 53ab1c64 ("IB/qib: Correct nfreectxts for multiple HCAs")
reversed the increments and decrements of dd->nfreectxts.  Fix it.

Reviewed-by: Ram Vepa <ram.vepa@qlogic.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@qlogic.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>