Staging: speakup: Fix return value in synth.c

The function return type is a pointer. Hence return NULL instead
of 0.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>