spi/bfin_spi: drop bits_per_word from client data

No other SPI controller has this field, and SPI clients should be setting
this up in their own drivers.  So drop it from the Blackfin controller to
keep people from using it.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Bob Liu <lliubbo@gmail.com>