cnic: Shutdown iSCSI ring during uio_close.

The iSCSI ring should be shutdown during uio_close instead of uio_open
for proper operations.  This fixes the problem of the ring getting
stuck intermittently.

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