cros_ec_spi makes the assumption that a 0-length message will put the
spi chip select back to normal (non cs_toggle mode).  This used to be
the case back on kernel-3.8 on the spi-s3c64xx driver but doesn't
appear to be true anymore.  It seems like it was a pretty questionable
assumption to begin with, so let's fix the code to be more robust.  We
know that a message with a single 0-length segment _will_ put things
back in order.  Change cros_ec_spi to handle this.

This wasn't a problem on the main user of cros_ec_spi upstream (tegra)
because it specified 'google,cros-ec-spi-msg-delay'.
Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>