IB/qib: Completion queue callback needs to be single threaded

Workqueues aren't exactly equivalent to tasklets since the callback
function may be called from multiple CPUs before the callback returns.
This causes completion notification callbacks to have MT bugs since
they weren't expecting this behavior. The fix is to use a single
threaded work queue.

Signed-off-by: Ralph Campbell <ralph.campbell@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>