mmc: core: Clarify that the ->ack_sdio_irq() callback is mandatory

For the MMC_CAP2_SDIO_IRQ_NOTHREAD case and when using sdio_signal_irq(),
the ->ack_sdio_irq() is already mandatory, which was not the case for those
host drivers that called sdio_run_irqs() directly.

As there are no longer any drivers calling sdio_run_irqs(), let's clarify
the code by dropping the unnecessary check and explicitly state that the
callback is mandatory in the header file.

Tested-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

include/linux/mmc/host.h

@@ -128,6 +128,7 @@ struct mmc_host_ops {
 	int	(*get_cd)(struct mmc_host *host);
 
 	void	(*enable_sdio_irq)(struct mmc_host *host, int enable);
+	/* Mandatory callback when using MMC_CAP2_SDIO_IRQ_NOTHREAD. */
 	void	(*ack_sdio_irq)(struct mmc_host *host);
 
 	/* optional callback for HC quirks */