mmc: sdhci-pci: fix 'Invalid iomem size' error message condition

The SDHCI standard defines a 256 byte register set but a device
that specifies a larger iomem region is not an error.  Alter the
message condition accordingly.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>