genirq: fix devres.o build for GENERIC_HARDIRQS=n

kernel/irq/devres.c is built by sparc (32bit) and m68k via the obscure
../../../kernel/irq/devres.o reference in arch/[sparc/m68k]/kernel/Makefile

To avoid ifdeffery in devres.c provide request_threaded_irq as an
inline for these users.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>