mmc: host: add note that set_ios needs to handle 0Hz properly

	void	(*pre_req)(struct mmc_host *host, struct mmc_request *req,

			   bool is_first_req);

	void	(*request)(struct mmc_host *host, struct mmc_request *req);

	/*

	 * Avoid calling the next three functions too often or in a "fast

	 * path", since underlaying controller might implement them in an

	 * expensive and/or slow way. Also note that these functions might

	 * sleep, so don't call them in the atomic contexts!

	 */

	/*

	 * Notes to the set_ios callback:

	 * ios->clock might be 0. For some controllers, setting 0Hz

	 * as any other frequency works. However, some controllers

	 * explicitly need to disable the clock. Otherwise e.g. voltage

	 * switching might fail because the SDCLK is not really quiet.

	 */

	void	(*set_ios)(struct mmc_host *host, struct mmc_ios *ios);

	/*

	 * Avoid calling these three functions too often or in a "fast path",

	 * since underlaying controller might implement them in an expensive

	 * and/or slow way.

	 *

	 * Also note that these functions might sleep, so don't call them

	 * in the atomic contexts!

	 *

	 * Return values for the get_ro callback should be:

	 *   0 for a read/write card

	 *   1 for a read-only card

	 *   -ENOSYS when not supported (equal to NULL callback)

	 *   or a negative errno value when something bad happened

	 *

	 */

	int	(*get_ro)(struct mmc_host *host);

	/*

	 * Return values for the get_cd callback should be:

	 *   0 for a absent card

	 *   1 for a present card

	 *   -ENOSYS when not supported (equal to NULL callback)

	 *   or a negative errno value when something bad happened

	 */

	void	(*set_ios)(struct mmc_host *host, struct mmc_ios *ios);

	int	(*get_ro)(struct mmc_host *host);

	int	(*get_cd)(struct mmc_host *host);

	void	(*enable_sdio_irq)(struct mmc_host *host, int enable);