Fixed dissection of 8-byte device descriptors

during usb device enumeration, a host may attempt to only read the
first 8 bytes of a device descriptor. Dissecting a partial device
descriptor was throwing a bounds error.

Change-Id: I09cdd356d15153afc93ee478fdd98329495a642a
Reviewed-on: https://code.wireshark.org/review/4137
Reviewed-by: Evan Huus <eapache@gmail.com>