input: synaptics_dsx: change permission for sysfs secure_touch_enable

Sysfs secure_touch_enable has permission 666. Remove write access
for "others" to avoid security concerns.

CRs-fixed: 763326
Change-Id: Ifcc34d418fb14af21440d2dd7200d125b02f85dd
Signed-off-by: Mao Li <maol@codeaurora.org>