spi: cadence: Return the error code for cdns_spi_suspend and cdns_spi_resume

Return the error code for cdns_spi_suspend and cdns_spi_resume.
Also fixes a comment where which claims that the error code is
returned.

Signed-off-by: Shubhrajyoti Datta <shubhraj@xilinx.com>
Signed-off-by: Mark Brown <broonie@kernel.org>