mmc: core: Avoid hogging the CPU while polling for busy for mmc ioctls

When __mmc_blk_ioctl_cmd() calls card_busy_detect() to verify that the
card's states moves back into transfer state, the polling with CMD13 is
done without any delays in between the commands being sent.

Rather than fixing card_busy_detect() in this regards, let's instead
convert into using the common mmc_poll_for_busy(), which also helps us to
avoid open-coding.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Link: https://lore.kernel.org/r/20210702134229.357717-3-ulf.hansson@linaro.org