spi/bfin_spi: convert read/write/duplex funcs to a dedicated ops structure

Rather than having to look up the same 3 sets of functions at the same
time, just use an ops structure so we only need to set one pointer.

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