mmc: sdhci-acpi: Get UID directly from acpi_device

UID was made available on acpi_device since
commit ccf78040 ("ACPI: Add _UID support for ACPI devices.")
Use it from there instead of reprocessing the
ACPI object info.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>