ugen panic fixes in usb/97271

Would a commiter with some USB knowledge please take a look at the 
patch in following PR.

http://www.freebsd.org/cgi/query-pr.cgi?pr=97271

iedowse hasn't returned any of my emails so I'm assuming he's just 
busy.  So if another commiter familiar with USB would take a look I'd 
appreciate any feedback.

Thanks,

-- 
Anish Mistry