spi/bfin_spi: drop custom cs_change_per_word support

As David points out, the cs_change_per_word option isn't standard, nor is
anyone actually using it.  So punt all of the dead code considering it
makes up ~10% of the code size.

Reported-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>