usb/106435: possible buffer overflow in ums(4) debug code

[Eygene Ryabinkin]

Gentlemen!

May I bother you with usb/106435 again. This PR fixes the issue with
ums packet length in the debug code: it was wrongly assumed that
packet will be 6 bytes long at each time. But since memory for the
buffer is allocated with the reported length and not with the constant
'6', then there is a potential buffer overflow. The fix is simple and
will not harm anything.

Thanks!
-- 
Eygene