USB: serial: fix return value for unsupported ioctls

Drivers should return -ENOTTY ("Inappropriate I/O control operation")
when an ioctl isn't supported, while -EINVAL is used for invalid
arguments.

Fix up the TIOCMGET, TIOCMSET and TIOCGICOUNT helpers which returned
-EINVAL when a USB serial driver did not implement the corresponding
methods.

Note that the TIOCMGET and TIOCMSET helpers predate git and do not get a
corresponding Fixes tag below.

Fixes: d281da7f ("tty: Make tiocgicount a handler")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/usb-serial.c

@@ -540,7 +540,7 @@ static int serial_tiocmget(struct tty_struct *tty)
 
 	if (port->serial->type->tiocmget)
 		return port->serial->type->tiocmget(tty);
-	return -EINVAL;
+	return -ENOTTY;
 }
 
 static int serial_tiocmset(struct tty_struct *tty,
@@ -552,7 +552,7 @@ static int serial_tiocmset(struct tty_struct *tty,
 
 	if (port->serial->type->tiocmset)
 		return port->serial->type->tiocmset(tty, set, clear);
-	return -EINVAL;
+	return -ENOTTY;
 }
 
 static int serial_get_icount(struct tty_struct *tty,
@@ -564,7 +564,7 @@ static int serial_get_icount(struct tty_struct *tty,
 
 	if (port->serial->type->get_icount)
 		return port->serial->type->get_icount(tty, icount);
-	return -EINVAL;
+	return -ENOTTY;
 }
 
 /*