mmc: jz4740: rework pre_req/post_req implementation

As reported by Aaro, the JZ4740 MMC driver throws a warning when the kernel
is built without preemption (CONFIG_PREEMPT_NONE=y).

[   16.461094] jz4740-mmc 13450000.mmc: [jz4740_mmc_prepare_dma_data] invalid cookie: data->host_cookie 567 host->next_data.cookie 568
[   16.473120] jz4740-mmc 13450000.mmc: [jz4740_mmc_prepare_dma_data] invalid cookie: data->host_cookie 568 host->next_data.cookie 569
[   16.485144] jz4740-mmc 13450000.mmc: [jz4740_mmc_prepare_dma_data] invalid cookie: data->host_cookie 569 host->next_data.cookie 570
[   16.497170] jz4740-mmc 13450000.mmc: [jz4740_mmc_prepare_dma_data] invalid cookie: data->host_cookie 570 host->next_data.cookie 571

The problem seems to be related to how pre_req/post_req is implemented.
Currently, it seems the driver expects jz4740_mmc_prepare_dma_data() to be
called with monotonically increasing host_cookie values, which is wrong.

Moreover, the implementation is overly complicated, keeping track of
unneeded "next cookie" state.

So, instead of attempting to fix the current pre_req/post_req
implementation, this commit refactors the driver, dropping the state,
following other drivers such as dw_mmc and sdhci.

Cc: Paul Cercueil <paul@crapouillou.net>
Cc: Mathieu Malaterre <malat@debian.org>
Reported-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>