net: qede: stop adding events on an already destroyed workqueue

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: Alexander Lobakin <alobakin@marvell.com>
Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: Michal Kalderon <michal.kalderon@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>