an endpoint_descriptor may only be sent from a USB device to the host
as part of a GET DESCRIPTOR Response

therefore, pinfo->destport==NO_ENDPOINT is always true for valid USB
packets and we can remove the else branch

Change-Id: I2163e3000cafa27113ee0e6173fd69b7ef17b812
Reviewed-on: https://code.wireshark.org/review/3585
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>