mmc: tmio: clear force_pio flag before starting data transfer

Currently, force_pio is cleared when the driver exits.  Then, it
resulted in clearing it in multiple places since MMC drivers in
general have multiple exit points.

 tmio_mmc_reset_work - bails out on timeout
 tmio_process_mrq - error out when it cannot send a command
 tmio_mmc_finish_request - successful exit

This is error-prone since we may miss to cover all bail-out points.

To simplify the code, the data structure should be initialized just
before used since we have a single entrance.  force_pio is only used
for data transfer, so tmio_mmc_start_data() will be a suitable place
to clear this flag.

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>