mmc: sdhci-pci: Allow deferred probe for sd card detect gpio

With commit f35bbf61 ("gpio / ACPI: Return -EPROBE_DEFER if the
gpiochip was not found"), a gpio descriptor request can now be deferred if
the providing gpio host controller driver hasn't been loaded yet. Allow use
in mmc slot probe in order to prevent card detect gpio setup from failing
in this case.

Signed-off-by: David E. Box <david.e.box@linux.intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>