PCI: Fix sriov_enable() error path for pcibios_enable_sriov() failures

Disable VFs if pcibios_enable_sriov() fails, just like we do for other
errors in sriov_enable().  Call pcibios_sriov_disable() if virtfn_add()
fails.

[bhelgaas: changelog, split to separate patch for reviewability]
Fixes: 995df527 ("PCI: Add pcibios_sriov_enable() and pcibios_sriov_disable()")
Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Wei Yang <weiyang@linux.vnet.ibm.com>