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>

drivers/staging/speakup/varhandlers.c

@@ -277,7 +277,7 @@ int spk_set_mask_bits(const char *input, const int which, const int how)
 			spk_chartab[*cp] &= ~mask;
 	}
 	cp = (u_char *)input;
-	if (cp == 0)
+	if (!cp)
 		cp = spk_punc_info[which].value;
 	else {
 		for ( ; *cp; cp++) {