mmc: sdhci-acpi: support deferred probe

With commit 67bf5156 ("gpio / ACPI: fix returned error from
acpi_dev_gpio_irq_get()"), mmc_gpiod_request_cd() returns -EPROBE_DEFER if
GPIO is not ready when sdhci-acpi driver is probed, and sdhci-acpi driver
should be probed again later in this case.

This fixes an order issue when both GPIO and sdhci-acpi drivers are built
as modules.

CC: stable@vger.kernel.org # v4.9
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=177101


Tested-by: Jonas Aaberg <cja@gmx.net>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>