pch_phub: return -ENODATA if ROM can't be mapped

The error return err is not initialized for the case when pci_map_rom
fails and no ROM can me mapped.  Fix this by setting ret to -ENODATA;
(this is the same error value that is returned if the ROM data is
successfully mapped but does not match the expected ROM signature.).

Issue found from static code analysis using CoverityScan.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>