when a linux USB isochronous descriptor is selected,
highlight its data part as well

Change-Id: Ia69ac6b1460245ffa44b6ad3b17f07948baf03d4
Reviewed-on: https://code.wireshark.org/review/3463
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>