RDMA/bnxt_re: Synchronize destroy_qp with poll_cq

Avoid system crash when destroy_qp is invoked while
the driver is processing the poll_cq. Synchronize these
functions using the cq_lock.

Signed-off-by: Selvin Xavier <selvin.xavier@broadcom.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>