mmc: pci_mmc.c should build with ACPIGEN=n

sandbox_defconfig builds the PCI MMC driver. It should be possible to
build the sandbox without ACPI support.

ACPI support in the PCI MMC driver is only needed when creating an ACPI
table. Fix building with ACPIGEN=n.

Fixes: dba7ee419d9d ("acpi: mmc: Generate ACPI info for the PCI SD Card")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/drivers/mmc/pci_mmc.c b/drivers/mmc/pci_mmc.c
index cba2ea8..9fb7044 100644
--- a/drivers/mmc/pci_mmc.c
+++ b/drivers/mmc/pci_mmc.c
@@ -86,6 +86,7 @@
 	return sdhci_bind(dev, &plat->mmc, &plat->cfg);
 }
 
+__maybe_unused
 static int pci_mmc_acpi_fill_ssdt(const struct udevice *dev,
 				  struct acpi_ctx *ctx)
 {
@@ -138,7 +139,9 @@
 }
 
 struct acpi_ops pci_mmc_acpi_ops = {
+#ifdef CONFIG_ACPIGEN
 	.fill_ssdt	= pci_mmc_acpi_fill_ssdt,
+#endif
 };
 
 static const struct udevice_id pci_mmc_match[] = {