sdhci: assertion in sdhci_read_dataport()
Description of problem:
The following log reveals it:

```
qemu-system-x86_64: qemu/hw/sd/sdhci.c:476: uint32_t sdhci_read_dataport(SDHCIState *, unsigned int): Assertion `s->data_count < s->buf_maxsz' failed.
Aborted (core dumped)
```