Currently, reading from the disp attribute fails with "No such device",
which is misleading. According to CMSG table on acpi4asus project site,
no models have a getter method corresponding to SDSP. Change the file
permission to disallow reads.

If some joker changes the permission to permit reads, then return -EIO
to be consistent with sysfs' behaviour when no show() method is
provided.
Signed-off-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Len Brown <len.brown@intel.com>