What UART baud rates are supported by the 60-series for downloading Bluetooth firmware to the module?

While the high speed UART of the 88W8997 chip on the 60-series supports up to 4 Mbaud the speed is actually fixed to 3 Mbaud in the firmware. It cannot be changed.

So, a typical download command looks like this:

hciattach /dev/ttyS1 any -s 3000000 3000000 flow dtron

While "ttyS1" of course has to be repalced by your actual Bluetooth UART.