spi: ep93xx: fix error return code in ep93xx_spi_probe()

Fix to return -ENOMEM in the workqueue create error handling
case instead of 0, as done elsewhere in this function.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>