irq: Kill pointless irqd_to_hw export

It makes no sense to export this trivial function.  Make it a static inline
instead.

This patch also drops virq_to_hw from arch/c6x since it is unused by that
architecture.

v2: Move irq_hw_number_t into types.h to fix ARM build failure

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>

include/linux/types.h

@@ -210,6 +210,12 @@ typedef u32 phys_addr_t;
 
 typedef phys_addr_t resource_size_t;
 
+/*
+ * This type is the placeholder for a hardware interrupt number. It has to be
+ * big enough to enclose whatever representation is used by a given platform.
+ */
+typedef unsigned long irq_hw_number_t;
+
 typedef struct {
 	int counter;
 } atomic_t;