The symbol rate in the Cable Delivery System Descriptor is in Mega-Symbols/s

Change-Id: I1446500f1c5c89b8ed94c04b04bb6985ae827731
Reviewed-on: https://code.wireshark.org/review/6095
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>