DragonFly has moved from the old USB stack to USB4BSD by default.  That means:

• If you are already using USB4BSD, you will want to remove WANT_USB4BSD from your kernel config.
• If you have trouble, switch back to the old USB.
• There’s some drivers that are not yet converted; help with them would be appreciated.  
• A full kernel/world build and ‘make upgrade’ will be needed in either case.

Sascha Wildner’s announcement email has all the gory details, including the kernel config changes to move back to the old USB setup.  This is of course in master; 3.6 users are unaffected.