spi/bfin_spi: fix handling of default bits per word setting

The default bits per word setting should be 8 bits, but since most of our
devices have been explicitly setting this up, we didn't notice when the
default stopped working.

At the moment, any default transfers without an explicit bit size setting
error out with:
bfin-spi bfin-spi.0: transfer: unsupported bits_per_word

So in the transfer logic, have a bits_per_word setting of 0 fall into the
8 bit transfer logic.

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