This might be the only option. I tried some hints with no luck. None listed in the sdhci(4) manual.the other 'proper' solution is to build a custom kernel without the device
hint.sdhci.0.disabled="1"
hint.sdhci.disabled="1"
hint.sdhci_pci0.disabled="1"
hint.sdhci_pci0.0.disabled="1"
hint.sdhci.0.disabled="YES"
hint.sdhci.disabled="YES"
hint.sdhci_pci0.disabled="YES"