device_add_child: protect against child device with no driver but fixed unit number
device_add_child: protect against child device with no driver but fixed unit number

This combination doesn't make sense, unit numbers should be hardwired
only in context of a known driver.  The wildcard devices should have
wildcard unit numbers.

Reviewed by:	jhb
MFC after:	2 weeks

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase