mmc: usdhi6rol0: Implement card_busy function

When switching card voltage to UHS voltage the mmc framework tries to
check the card busy signal, meaning the card pulling DAT0 line low,
before the switch is made. Drivers that does not implement the card_busy
function will manage to do the switch anyway, but the framework will
print a warning about not being able to verify the voltage signal.

Implement card_busy function.

Signed-off-by: Mårten Lindahl <marten.lindahl@axis.com>
Link: https://lore.kernel.org/r/20210816142314.1168-1-marten.lindahl@axis.com


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