spi: spi_bfin handles spi_transfer.cs_change

Respect per-transfer cs_change field (protocol tweaking support) by
adding and using cs_active/cs_deactive functions.

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>