cnic: Fine-tune ring init code.

Initialize the rings only during cnic_uio_open() and shut them down
during cnic_uio_close().  Check for the new bit CNIC_LCL_FL_RINGS_INITED
before checking for ring interrupt.

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