USB: cdc-acm: remove superfluous prototype

Remove unnecessary acm_tty_chars_in_buffer prototype.
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/class/cdc-acm.c

@@ -626,8 +626,6 @@ static void acm_tty_unregister(struct acm *acm)
 	kfree(acm);
 }
 
-static int acm_tty_chars_in_buffer(struct tty_struct *tty);
-
 static void acm_port_down(struct acm *acm)
 {
 	int i, nr = acm->rx_buflimit;