dmaengine: stedma40: add missing iounmap() on error in d40_probe()

Add the missing iounmap() before return from d40_probe()
in the error handling case.

Fixes: 8d318a50

 ("DMAENGINE: Support for ST-Ericssons DMA40 block v3")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20210518141108.1324127-1-yangyingliang@huawei.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>