Blackfin: convert DMA code to a proper bitmap

Rather than using our own data structures that basically boil down to a
bitmap, use the standard bitmap functions.

Reported-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>