Staging: speakup: serialio.c: Remove explicit NULL comparison

Remove explicit NULL comparison and write it in its simpler form.
Replacement done with coccinelle:

@replace_rule@
expression e;
@@

-e == NULL
+ !e

Signed-off-by: Shraddha Barke <shraddha.6596@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/speakup/serialio.c

@@ -101,7 +101,7 @@ static void start_serial_interrupt(int irq)
 {
 	int rv;
 
-	if (synth->read_buff_add == NULL)
+	if (!synth->read_buff_add)
 		return;
 
 	rv = request_irq(irq, synth_readbuf_handler, IRQF_SHARED,
@@ -127,7 +127,7 @@ void spk_stop_serial_interrupt(void)
 	if (speakup_info.port_tts == 0)
 		return;
 
-	if (synth->read_buff_add == NULL)
+	if (!synth->read_buff_add)
 		return;
 
 	/* Turn off interrupts */