mmc: rtsx: add helper function to simplify code

To support sdio card, transfer mode need make a change,
this need to split code and use it in different place,
Add new function to simplify repeat operation.

Signed-off-by: Micky Ching <micky_ching@realsil.com.cn>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>