USB: musb: enable low level DMA operation for Blackfin

- DMA registers in Blackfin have different layout
- DMA interrupt flags need to be cleared by software

Signed-off-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>