mmc: tmio: simplify the DMA mode test

host->chan_{rx,tx} represents the DMA capability of the platform.
Even if DMA is supported, there are cases where we want to use PIO,
for example, data length is short enough as commit 5f52c355
("mmc: tmio: use PIO for short transfers") mentioned.

Regarding the hardware control flow, we are interested in whether DMA
is currently enabled or not, instead of whether the platform has the
DMA capability.

Hence, the several conditionals in tmio_mmc_core.c end up with
checking host->chan_{rx,tx} and !host->force_pio. This is not nice.

Let's flip the flag host->force_pio into host->dma_on.

host->dma_on represents whether the DMA is currently enabled or not.
This flag is set false in the beginning of each command, then should
be set true by tmio_mmc_start_dma() when the DMA is turned on.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>