Comment 3 for bug 863799

Thanks to an ArchLinux forum thread, I discovered that the issue is with multiple drivers trying to manage the Broadcom chip. I'm using the restricted "wl" driver, but the opensource drivers load too.

To fix this, I added the following three lines to the bottom of /etc/modprobe.d/blacklist.conf -

blacklist bcma
blacklist brcmsmac
blacklist brcmutil

I'm not familiar with how the restricted drivers are distributed in Ubuntu - it seems that the long-term fix is to blacklist the opensource drivers when the restricted one is enabled.