Blackfin SPI Driver: unify duplicated code in dma read/write paths

For DMA TX/RX operation in pump_transfers, DMA contriguration code in TX
and RX paths are almost the same.  This patch unify the duplicated DMA
code to make it more readable.

Signed-off-by: Mike Frysinger <vapier.adi@gmail.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
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>