input: synaptics: Correct sysfs permissions

Set correct permissions for write-only sysfs nodes. Show method of such
attributes is set to NULL since the attributes are write-only.

Change-Id: Ic37b2c5c67f9e68f9a2f06e379e0aed5cdb756bd
CRs-Fixed: 573711
Signed-off-by: Himanshu Aggarwal <haggarwa@codeaurora.org>