drivers/rtc/rtc-ds1511.c: world-writable sysfs nvram file

Don't allow everybogy to write to NVRAM.

Signed-off-by: Vasiliy Kulikov <segoon@openwall.com>
Cc: Andy Sharp <andy.sharp@onstor.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>