Blackfin SPI Driver: add a few more DMA debug messages

Because of DMA hardware issue, we were trying to use software workaround.
This patch add some useful debug messages to help us debugging the DMA
code.

Signed-off-by: Mike Frysinger <vapier.adi@gmail.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>