mmc: core: comment on why sdio_reset is done at init time

sdio_reset sends a CMD52 to reset the sdio card.  This is highly
recommended for sdio cards being reinitialized.  Since we do not
know if the card is being reinitialized we just send the command.
SD/eMMC cards are supposed to ignore the CMD before the CMD0.
Document why we are doing this.

Signed-off-by: Philip Rakity <prakity@marvell.com>
Signed-off-by: Chris Ball <cjb@laptop.org>