mmc: meson: Fix usage of meson_mmc_post_req()

Currently meson_mmc_post_req() is called in meson_mmc_request() right
after meson_mmc_start_cmd(). This could lead to DMA unmapping before the request
is actually finished.

To fix, don't call meson_mmc_post_req() until meson_mmc_request_done().

Signed-off-by: Rong Chen <rong.chen@amlogic.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Fixes: 79ed05e3

 ("mmc: meson-gx: add support for descriptor chain mode")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20220216124239.4007667-1-rong.chen@amlogic.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>