waveguide: log stations on secondary APs.

This is important now that we have a guest network with devices
remaining on the secondary _portal interface for extended periods of
time.

Prefix log lines for connected stations with the interface they are
connected to, since this is meaningful now from a business standpoint.
The log lines look like:

wlan0_portal(aa:15:62:43:20:0c): Connected station 80:7a:bf:6f:79:bd taxonomy: SHA:d00571e4b38df159b363b2ce05d89f398aceca18944f50b0ed1ce3804f9ded8b;Unknown;802.11n n:2,w:20

BUG=31601144

Change-Id: I25aed0ad24fe0d1906481e79fc6dbd9fa79ca9a9
4 files changed