mmc: mmci: Unwind big if() clause

mmc: mmci: Unwind big if() clause

This does two things: firsr replace the hard-to-read long
if-expression:

  if (!host->busy_status && !(status & err_msk) &&
      (readl(base + MMCISTATUS) & host->variant->busy_detect_flag)) {

With the more readable:

  if (!host->busy_status && !(status & err_msk)) {
     status = readl(base + MMCISTATUS);
     if (status & host->variant->busy_detect_flag) {

Second notice that the re-read MMCISTATUS register is now
stored into the status variable, using logic OR because what
if something else changed too?

While we are at it, explain what the function is doing.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20230405-pl180-busydetect-fix-v7-2-69a7164f2a61@linaro.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>