[ARM] S3C24XX: Fix unused code warning in arch/arm/plat-s3c24xx/dma.c

Fix unused code warning in arch/arm/plat-s3c24xx/dma.c if there
is no PM support enabled. The function to_dma_chan() should
be marked inline so that the compiler will eliminate it without
warning if it isn't used.

arch/arm/plat-s3c24xx/dma.c:1239: warning: 'to_dma_chan' defined but not used

Signed-off-by: Ben Dooks <ben-linux@fluff.org>