scsi: myrs: Fix a double free in myrs_cleanup()

commit 2bb81771 upstream.

In myrs_cleanup(), cs->mmio_base will be freed twice by iounmap().

Link: https://lore.kernel.org/r/20210311063005.9963-1-lyl2019@mail.ustc.edu.cn
Fixes: 77266186

 ("scsi: myrs: Add Mylex RAID controller (SCSI interface)")
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Lv Yunlong <lyl2019@mail.ustc.edu.cn>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>