mmc: mmci: simplify err check in mmci_post_request

The error condition indicates that mmci_post_request() should cleanup
after the mmci_pre_request(). In this case the resources allocated by
device_prep_slave_sg() are freed by calling dmaengine_terminate_all().
dma_unmap_sg() should always be performed if the host_cookie is set.

Signed-off-by: Per Forlin <per.forlin@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>