Give the fdt helper routines static linkage since no global definition
of them is provided anywhere.  (gcc was nice enough to warn about this,
clang didn't for some reason.)
In sys/dev/usb/controller/uss820dci.c, similar to r261977, fix a warning
about uss820dci_odevd being unused, by adding it to the part that
handles getting descriptors.

Reported by:	loos
Reviewed by:	hselasky
MFC after:	3 days