Blackfin SPI Driver: use len_in_bytes when we care about the number of bytes transferred

Use len_in_bytes when we care about the number of bytes transferred rather
than the number of spi transactions.  (this value will be the same for
8bit transfers, but not any other sizes)

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>