spi: Return error from of_spi_register_master on bad "cs-gpios" property

This makes sure that an error is returned on an incorrectly formed
"cs-gpios" property, but reports success when the "cs-gpios" property is
well formed or missing.

When holes in the cs-gpios property phandle list is used to indicate
that some other form of chipselect is to be used it is important that
failure to read a broken "cs-gpios" property does not silently fail
leading to the spi controller to use an unintended chipselect.

Signed-off-by: Andreas Larsson <andreas@gaisler.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>