RDMA/cxgb3: Don't pass irq flags to flush_qp()

Since flush_qp() is always called with irqs disabled, all the locking
inside flush_qp() and __flush_qp() doesn't need irq save/restore.

Further, passing the flag variable from iwch_modify_qp() is just wrong
and causes a WARN_ON() in local_bh_enable().

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>