mmc: core: Re-work HW reset for SDIO cards

It have turned out that it's not a good idea to unconditionally do a power
cycle and then to re-initialize the SDIO card, as currently done through
mmc_hw_reset() -> mmc_sdio_hw_reset(). This because there may be multiple
SDIO func drivers probed, who also shares the same SDIO card.

To address these scenarios, one may be tempted to use a notification
mechanism, as to allow the core to inform each of the probed func drivers,
about an ongoing HW reset. However, supporting such an operation from the
func driver point of view, may not be entirely trivial.

Therefore, let's use a more simplistic approach to solve the problem, by
instead forcing the card to be removed and re-detected, via scheduling a
rescan-work. In this way, we can rely on existing infrastructure, as the
func driver's ->remove() and ->probe() callbacks, becomes invoked to deal
with the cleanup and the re-initialization.

This solution may be considered as rather heavy, especially if a func
driver doesn't share its card with other func drivers. To address this,
let's keep the current immediate HW reset option as well, but run it only
when there is one func driver probed for the card.

Finally, to allow the caller of mmc_hw_reset(), to understand if the reset
is being asynchronously managed from a scheduled work, it returns 1
(propagated from mmc_sdio_hw_reset()). If the HW reset is executed
successfully and synchronously it returns 0, which maintains the existing
behaviour.

Reviewed-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Douglas Anderson <dianders@chromium.org>
Cc: stable@vger.kernel.org # v5.4+
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>