spi: Add error checking for invalid bus widths

spi: Add error checking for invalid bus widths

At present an invalid bus width prints a message but does not return an
error. This is the opposite of the correct behaviour. Adjust it to avoid
code bloat in the common case, and avoid hard-to-debug failure in the
uncommon case.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jagan@openedev.com>