mfd: ab3100: world-writable debugfs *_priv files

Don't allow everybody to change device hardware registers.

Signed-off-by: Vasiliy Kulikov <segoon@openwall.com>
Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>