[IA64] Fix warning in dma-mapping.c

arch/ia64/kernel/dma-mapping.c:14: warning: control reaches end of non-void function
arch/ia64/kernel/dma-mapping.c:14: warning: no return statement in function returning non-void

This warning was introduced by commit: 390bd132
	Add dma_debug_init() for ia64

Signed-off-by: Tony Luck <tony.luck@intel.com>