Blackfin: convert DMA mutex to an atomic and drop redundant code

The DMA channel status field was encoding redundant info wrt the DMA MMR
config register, and it was doing an incomplete job of checking all DMA
channels (some drivers write directly to the config register).  So drop
the tristate field in favor of a binary atomic field.  This simplifies
the code in general, removes the implicit need for sleeping, and forces
the suspend code to handle all channels properly.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>