mmc: core: Factor out some of the code in mmc_go_idle()

Move some part out from mmc_go_idle() into a new function called
__mmc_go_idle(), allowing it to be re-used, which is shown from a
subsequent change.

Signed-off-by: Victor Shih <victor.shih@genesyslogic.com.tw>
Link: https://lore.kernel.org/r/20240913102836.6144-7-victorshihgli@gmail.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>