mmc: core: Enable re-use of mmc_blk_in_tran_state()

To allow subsequent changes to re-use the code from the static function
mmc_blk_in_tran_state(), let's move it to a public header. While at it,
let's also rename it to mmc_ready_for_data(), as to try to better describe
its purpose.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Baolin Wang <baolin.wang7@gmail.com>
Tested-by: Ludovic Barre <ludovic.barre@st.com>
Reviewed-by: Ludovic Barre <ludovic.barre@st.com>
Link: https://lore.kernel.org/r/20200204085449.32585-7-ulf.hansson@linaro.org