ab8500-chargalg: Only root should have write permission on sysfs file

commit e3455002d04276c256a531f7175dce0f7d1cb78a upstream.

Only root should have write permission on sysfs file ab8500_chargalg/chargalg.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>