drivers/scsi/pm8001/pm8001_ctl.c: avoid world-writable sysfs files.

In line with practice for module parameters, we're adding a build-time
check that sysfs files aren't world-writable.

Cc: Lindar Liu <lindar_liu@usish.com>
Cc: James Bottomley <JBottomley@Parallels.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>