Input: atmel_mxt_ts - only allow root to update firmware

Restrict permissions on the update_fw sysfs entry to read only for root
only.

Also, update object permission to use a macro S_IRUGO macro instead of
hard coded 0444.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Joonyoung Shim <jy0922.shim@samsung.com>
Acked-by: Nick Dyer <nick.dyer@itdev.co.uk>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>