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: NDaniel Kurtz <djkurtz@chromium.org>
Reviewed-by: NJoonyoung Shim <jy0922.shim@samsung.com>
Acked-by: NNick Dyer <nick.dyer@itdev.co.uk>
Signed-off-by: NDmitry Torokhov <dtor@mail.ru>