staging: skein: Fixing single statement macro checkpatch warning

WARNING: Single statement macros should not use a do {} while (0) loop

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>