powerpc/pci: Fix SRIOV not building without EEH enabled

On Book3E CPUs (and possibly other configs), it is possible to have SRIOV
(CONFIG_PCI_IOV) set without CONFIG_EEH.  The SRIOV code does not check
for this, and if EEH is disabled, pci_dn.c fails to build.

Fix this by gating the EEH-specific code in the SRIOV implementation
behind CONFIG_EEH.

Fixes: 39218cd0

 ("powerpc/eeh: EEH device for VF")
Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>