cnic: Return SPQ credit to bnx2x after ring setup and shutdown.

Everytime the iSCSI ring finishes setup or shutdown, we need to return
the SPQ (slow path queue) credit to the bnx2x driver.  Without this step,
the SPQ will eventually be full causing iSCSI to fail.  This can happen
after 3 or 4 MTU changes for example.

Add code to wait for these slow path commands to complete in the RX ring
and return the SPQ credit to bnx2x.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>