Blackfin arch: set_dma_callback: do not store .irq

set_dma_callback: do not store .irq if request_irq() failed so we dont
turn around and attempt to free_irq() it later on in free_dma()

Signed-off-by: Mike Frysinger <vapier.adi@gmail.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>