Fix rtu request pdu length calculation

[AsgerWenneb]

PR to fix the request PDU length calculation mentioned in https://github.com/slowtec/modbus-core/issues/12.

My application doesn't use coils but I've attempted to implement the fix for FC 0x0F as well. I have not looked at the TCP implementation at all (I have no experience with it). An error was also added for when the quantity and bytes fields don't match up.