mmc: meson-gx: make sure the descriptor is stopped on errors

On errors, if we don't stop the descriptor chain, it may continue to
run and raise IRQ after we have called mmc_request_done(). This is bad
because we won't be able to get cmd anymore and properly deal with the
IRQ.

This patch makes sure the descriptor chain is stopped before
calling mmc_request_done()

Fixes: 79ed05e3

 ("mmc: meson-gx: add support for descriptor chain mode")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>