mmc: sdhci-pci: Fix error case in sdhci_pci_probe_slot()

If pci_ioremap_bar() fails during probe, we "goto release;" and free the
host, but then we return 0 -- which tells sdhci_pci_probe() that the probe
succeeded.  Since we think the probe succeeded, when we unload sdhci we'll
go to sdhci_pci_remove_slot() and it will try to dereference slot->host,
which is now NULL because we freed it in the error path earlier.

The patch simply sets ret appropriately, so that sdhci_pci_probe() will
detect the failure immediately and bail out.

Signed-off-by: Chris Ball <cjb@laptop.org>
Cc: <stable@kernel.org>