mmc: core: Update ->card_busy() callback comment

According to SD specification checking state of DAT0 only, is enough while
polling for card busy completion. Let's update the comment in the header
file to correct this, as the comment says DAT[0:3].

Signed-off-by: Mårten Lindahl <marten.lindahl@axis.com>
Link: https://lore.kernel.org/r/20210816153054.24082-1-marten.lindahl@axis.com


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