iw_cxgb4: Fix qpid leak

Add await in destroy_qp() so that all references to qp are dereferenced
and qp is freed in destroy_qp() itself.  This ensures freeing of all QPs
before invocation of dealloc_ucontext(), which prevents loss of in use
qpids stored in the ucontext.

Signed-off-by: Nirranjan Kirubaharan <nirranjan@chelsio.com>
Reviewed-by: Potnuri Bharat Teja <bharat@chelsio.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>