powerpc/eeh: Null check uses of eeh_pe_bus_get

eeh_pe_bus_get() can return NULL if a PCI bus isn't found for a given PE.
Some callers don't check this, and can cause a null pointer dereference
under certain circumstances.

Fix this by checking NULL everywhere eeh_pe_bus_get() is called.

Fixes: 8a6b1bc7 ("powerpc/eeh: EEH core to handle special event")
Cc: stable@vger.kernel.org # v3.11+
Signed-off-by: Russell Currey <ruscur@russell.cc>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>