Blackfin SPI Driver: do not check for SPI errors if DMA itself did not flag any

We only need to check SPI error when DMA failes, cause that is the DMA IRQ
handling routine.

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>