Fix string bitrate to unit and value conversion

@fmessaoudi @tien-thinh.nguyen this MR is the fix for the MBR value issue.

The MBR issue was due to uint_16 limitation and a bug in the conversion function.

If bitrate value is greater than uint_16 max, we reduce the value and increase the unit.

If reducing leads to cut of values (like 123.450 Kbps -> 123 Mbps) we now send a warning but cut it off anyway.