mmc: core: Move code to get pending SDIO IRQs to a function

To improve code quality, let's move the code that gets pending SDIO IRQs
from process_sdio_pending_irqs() into a dedicated function.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
[Ulf: Converted function into static]
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>