dma-debug: fix dma_debug_add_bus() definition for !CONFIG_DMA_API_DEBUG

Impact: build fix

Fix:

 arch/x86/kvm/x86.o: In function `dma_debug_add_bus':
 (.text+0x0): multiple definition of `dma_debug_add_bus'

dma_debug_add_bus() should be a static inline function.

Cc: Joerg Roedel <joerg.roedel@amd.com>
LKML-Reference: <20090317120112.GP6159@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>