[SCSI] megaraid_sas: remove sysfs poll_mode_io world writeable permissions

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

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

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>