fix wrong offset for ID Capability Flags 1
reported by Jean-Marc Lamond

Bug: 10597
Change-Id: I481fad9cd0b8d55e3dd3af00cde2897105547f13
Reviewed-on: https://code.wireshark.org/review/4898
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>