s390/smsgiucv: disable SMSG on module unload

[ Upstream commit 760dd0eeaec1689430243ead14e5a429613d8c52 ]

The module exit function of the smsgiucv module uses the incorrect CP
command to disable SMSG messages. The correct command is "SET SMSG OFF".
Use it.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>