um: Implement um_free_irq()

Instead of using chip->release() we can achieve the same
using a simple wrapper for free_irq().
We have already um_request_irq(), so um_free_irq() is the perfect
counterpart.

Signed-off-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

arch/um/drivers/xterm_kern.c

@@ -65,7 +65,7 @@ int xterm_fd(int socket, int *pid_out)
 	 * isn't set) this will hang... */
 	wait_for_completion(&data->ready);
 
-	free_irq(XTERM_IRQ, data);
+	um_free_irq(XTERM_IRQ, data);
 
 	ret = data->new_fd;
 	*pid_out = data->pid;