mmc: core: Split up mmc_poll_for_busy()

To make the code more readable, move the part that gets the busy status of
the card out into a separate function, mmc_busy_status(). Then call it from
mmc_poll_for_busy().

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/20200204085449.32585-6-ulf.hansson@linaro.org