Don't assume all children of a nexus are ports.

[jhb]

Summary

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.

Test Plan

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