[SCSI] megaraid_sas: remove sysfs dbg_lvl world writeable permissions

/sys/bus/pci/drivers/megaraid_sas/dbg_lvl defaults to being
world-writable, which seems bad (letting any user affect kernel driver
behavior and logging level).

This turns off group and user write permissions, so that on typical
production systems only root can write to it.

[jejb: fix up rejections]
Signed-off-by: Joseph Malicki <jmalicki@metacarta.com>
Acked-by: "Yang, Bo" <Bo.Yang@lsi.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>