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/include/shared/irq_kern.h

@@ -13,6 +13,6 @@ extern int um_request_irq(unsigned int irq, int fd, int type,
 			  irq_handler_t handler,
 			  unsigned long irqflags,  const char * devname,
 			  void *dev_id);
-
+void um_free_irq(unsigned int irq, void *dev);
 #endif