staging: dgnc: change style of NULL comparison

Change the NULL comparison style as warned by checkpatch.
Signed-off-by: NSudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/dgnc/dgnc_tty.c

@@ -1553,7 +1553,7 @@ static int dgnc_tty_chars_in_buffer(struct tty_struct *tty)
 	uint chars = 0;
 	unsigned long flags;
 
-	if (tty == NULL)
+	if (!tty)
 		return 0;
 
 	un = tty->driver_data;
@@ -1658,7 +1658,7 @@ static int dgnc_tty_write_room(struct tty_struct *tty)
 	int ret = 0;
 	unsigned long flags;
 
-	if (tty == NULL || dgnc_TmpWriteBuf == NULL)
+	if (!tty || !dgnc_TmpWriteBuf)
 		return 0;
 
 	un = tty->driver_data;
@@ -1739,7 +1739,7 @@ static int dgnc_tty_write(struct tty_struct *tty,
 	ushort tmask;
 	uint remain;
 
-	if (tty == NULL || dgnc_TmpWriteBuf == NULL)
+	if (!tty || !dgnc_TmpWriteBuf)
 		return 0;
 
 	un = tty->driver_data;