input: synaptics_dsx_2.6: correct sysfs permissions

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

CRs-Fixed: 996733
Change-Id: I93b14e22da57a8a47a02080a05a9b8c666970272
Signed-off-by: Abinaya P <abinayap@codeaurora.org>