Comment # 12 on bug 1220909 from Ryan Stucki

I figured it out...

The Xbox driver module is "xpad" (from what I can tell)
The output of "lsmod | grep xpad" showed no results.

Running "modprobe xpad" fixed all issues.  Xbox controller now works, and Steam
controller no longer disconnects.  (It seems the Steam controller is detected
as an Xbox controller in Linux, so lack of Xbox driver was causing the issues.)

digging in further, there was a blacklist file in /etc/modprobe.d called
"xone-blacklist.conf"  This file blacklisted the "xpad" driver.

I commented out the line blacklisting the "xpad" driver, and everything works
well from boot now.

I have no idea how/when the blacklist file was created.  I'm guessing a recent
update switched to the "xone" driver instead of xpad, but xone doesn't seem to
be working...  ("lsmod | grep xone" shows no results as well..., and "modprobe
xone" spits back an error.)

WORKAROUND: "modprobe xpad"