dmaengine: ste_dma40: fix error return code in d40_probe()

In many of the error handling case, the return value 'ret' not set
and 0 will be return from d40_probe() even if error, but we should
return a negative error code instead in those error handling case.
This patch fixed them, and also removed useless variable 'err'.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>