mmc: core: Enable __mmc_switch() to change bus speed timing for the host

In cases when a speed mode change is requested for mmc cards, a CMD6 is
sent by calling __mmc_switch() during the card initialization. The CMD6
leads to the card entering a busy period. When that is completed, the host
must parse the CMD6 status to find out whether the change of the speed mode
succeeded.

To enable the mmc core to poll the card by using CMD13 to find out when the
busy period is completed, it's reasonable to make sure polling is done by
having the mmc host and the mmc card, being configured to operate at the
same selected bus speed timing.

Therefore, let's extend __mmc_switch() to take yet another parameter, which
allow its callers to update the bus speed timing of the mmc host. In this
way, __mmc_switch() also becomes capable of reading and validating the CMD6
status by sending a CMD13, in cases when that's desired.

If __mmc_switch() encounters a failure, we make sure to restores the old
bus speed timing for the mmc host, before propagating the error code.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>