powerpc/eeh: Reworked eeh_pe_bus_get()

The original implementation is ugly: unnecessary if statements and
"out" tag. This reworks the function to avoid above weaknesses. No
functional changes introduced.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>