Staging: speakup: Fix sparse warning in varhandlers.c

'cp' is a pointer. Fix the incorrect comparison with integer to
avoid the below warning:
drivers/staging/speakup/varhandlers.c:283:19: warning:
Using plain integer as NULL pointer

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>