PCI: ibmphp: Fix use-before-set in get_max_bus_speed()

The "rc" variable is only initialized on the error path.  The caller
doesn't check the return but, if "rc" is non-zero, then this function is
basically a no-op.

Fixes: 3749c51a ("PCI: Make current and maximum bus speeds part of the PCI core")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>