ucom debug not working on releng/14

A Pi2 running armv7 releng/14 reports

sysctl: unknown oid 'hw.usb.ucom.debug' at line 10

when booting. 

The intent was to understand why it drops ssh connections
when they are used to communicate with an ftdi usb-serial
adapter.  A few other hosts running -current accept the
debug setting and generate copious output in response.

Man pages suggest the option is available on stable,
-current and releng/, but it seems to actually work
only on -current, at least for armv7.

Thanks for reading, and any hints what might be done wrong.

bob prohaska