mmc: core: Add helper function to indicate if SDIO IRQs is enabled

To avoid each host driver supporting SDIO IRQs, from keeping track
internally about if SDIO IRQs has been claimed, let's introduce a common
helper function, sdio_irq_claimed().

The function returns true if SDIO IRQs are claimed, via using the
information about the number of claimed irqs. This is safe, even without
any locks, as long as the helper function is called only from
runtime/system suspend callbacks of the host driver.

Tested-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>