Specifically, ccr(4) devices are also children of cxgbe nexus devices.
Rather than making assumptions about the child device's softc, walk the
list of ports from the nexus' softc to determine if a child is a port
in t4_child_location_str(). This fixes a panic when detaching a ccr
device.

• kldunload of ccr currently panicks
• after change, kldunload no longer panicks, and no location info shows up for ccr0 in 'devinfo -v'