vfio/pci: Fix error return code in vfio_ecap_init()

The error code returned from vfio_ext_cap_len() is stored in 'len', not
in 'ret'.

Fixes: 89e1f7d4

 ("vfio: Add PCI device driver")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Reviewed-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Message-Id: <20210515020458.6771-1-thunder.leizhen@huawei.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>