ARM: S3C64XX: Tidy definition and comments in s3c_dma_has_circular()

The recent changes to arch/arm/mach-s3c6400/include/mach/dma.h have
left an out of date comment in there as well as accidentally changing
the type of the function.

Fix the commit 54489cd4

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