Set rdma_wq pointer to NULL after destroying the workqueue and check
for it when adding new events to fix crashes on driver unload.

Fixes: cee9fbd8 ("qede: Add qedr framework")
Signed-off-by: NAlexander Lobakin <alobakin@marvell.com>
Signed-off-by: NIgor Russkikh <irusskikh@marvell.com>
Signed-off-by: NMichal Kalderon <michal.kalderon@marvell.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>