staging: gasket: Replace macro __ATTR with __ATTR_NULL

Replace macro __ATTR with __ATTR_NULL since the former one is
unnecessary for gasket_sysfs_attr array terminator. Also silence the
checkpatch.pl warning:
WARNING:Symbolic permissions 'S_IRUGO' are not preferred. Consider using
octal
permissions '0444'.

Signed-off-by: Wentao Cai <etsai042@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>