spi_bfin: wait for tx to complete on full duplex paths

Full duplex SPI operation should not read a dummy byte at the first transfer.
Bug and fix by Jean-Christian de Rivaz <jc@eclis.ch>:

http://blackfin.uclinux.org/gf/project/uclinux-dist/tracker/?action=TrackerItemEdit&tracker_item_id=3678

Signed-off-by: Jean-Christian de Rivaz <jc@eclis.ch>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>