mmc: meson-gx: improve setting data->bytes_xfered

Centralize setting data->bytes_xfered in one place and make sure
that the number of transfererd bytes is set only if we receive
the end-of-chain interrupt and there was no error.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>