Comment 17 for bug 1553688

"mmc0: Unknown controller version (3)" comes from the kernel source code at:

linux/drivers/mmc/host/sdhci.c

It says mmc0 controller version is 3, which is greater than the version prescribed in the reported kernel.
In the latest mainline kernel, I can see:

#define SDHCI_SPEC_420 5

        if (host->version > SDHCI_SPEC_420) {
                pr_err("%s: Unknown controller version (%d). You may experience problems.\n",
                       mmc_hostname(mmc), host->version);
        }

This will also expect mmc controller version as 3.
This should be fixed if the ubuntu maintainer applies https://www.lkml.org/lkml/2018/8/29/365
in the next release.