scsi: stex: Fix reboot_notifier leak in probe error path

scsi: stex: Fix reboot_notifier leak in probe error path

In stex_probe(), register_reboot_notifier() is called at the beginning,
but if any subsequent initialization step fails, the function returns
without unregistering the notifier, resulting in a resource leak.

Add unregister_reboot_notifier() in the out_disable error path to ensure
proper cleanup on all failure paths.

Fixes: 61b745fa63db ("scsi: stex: Add S6 support")
Signed-off-by: Haotian Zhang <vulab@iscas.ac.cn>
Link: https://patch.msgid.link/20251104094847.270-1-vulab@iscas.ac.cn
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>