PCI: sysfs: Fix failure path for addition of "vpd" attribute

Commit 280c73d3 ("PCI: centralize the capabilities code in
pci-sysfs.c") changed the initialisation of the "rom" and "vpd"
attributes, and made the failure path for the "vpd" attribute
incorrect.  We must free the new attribute structure (attr), but
instead we currently free dev->vpd->attr.  That will normally be NULL,
resulting in a memory leak, but it might be a stale pointer, resulting
in a double-free.

Found by inspection; compile-tested only.

Cc: stable@kernel.org
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>