staging: speakup: remove NULL comparison

This was done using cocccinelle script:
@@
identifier arg;
@@

-arg==NULL
+!arg

Signed-off-by: Aastha Gupta <aastha.gupta4104@gmail.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>