Add more baud rate options when creating a custom Comm Link

[sseshan7]

Tell us a bit about the feature:

• What problem does it solve? Create custom Comm Links with telemetry radio devices that support faster baudrates than the current maximum of 921600.
• What does it do? Add 1200000 and 3000000 to the baud rate dropdowns. Or remove the dropdown completely and allow someone to type in the baudrate.
• Are there other systems that have this feature? RFD900 radios support up to 3000000 baud.

[booo]

You can try changing the list of available baud rates here:

https://github.com/mavlink/qgroundcontrol/blob/master/src/comm/SerialLink.cc#L412

[tpwrules]

I need a baud rate of 1000000. On my system 921600 falls at a bad divisor and doesn't work properly.