spi/bfin_spi: support for multiples of 8bits with hardware CS

We can do multiples of 8bit transfers when using the hardware CS and a
little bit of magic, so make it work.

Signed-off-by: Bob Liu <lliubbo@gmail.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>