Feature request: allow bumble-rtk-util to work with serial transport

I have a Realtek [dev board](https://www.alibaba.com/product-detail/realtek-RTL8761BTV-dongle-Blue-tooth-development_1600150205224.html) which exposes the chip via serial rather than USB. I confirmed on Windows that `bumble-rtk-util info usb:2550:8761` and `bumble-rtk-util load usb:2550:8761` work on a different normal USB dongle, to confirm it's not having the same issue as #746. And then I want to issue `bumble-rtk-util info serial:COM6,115200` and `bumble-rtk-util load serial:COM6,115200` on the dev board, to take advantage of the fact that Bumble supports HCI over serial (so I don't have to implement firmware download myself).

However, when I do `bumble-rtk-util info serial:COM6,115200` I get an error ["USB device not supported by this RTK driver"](https://github.com/google/bumble/blob/main/tools/rtk_util.py#L109) (caused by the `check()` function failing because there is of course no USB metadata when I'm giving a serial transport.)

Interestingly, if I add `--force` it gets around this and works for `info` and `load`, despite the fact that the code is written in such a way as to [assume USB transport](https://github.com/google/bumble/blob/main/tools/rtk_util.py#L99).

However ultimately the `--force` with `load` (which is what I really care about) is unsuccessful. It seems to me based on serial logging like it's padding out the end of the last 0xFC20 VSC download command and including a lot of extra 0 bytes despite the fact that the default `rtl8761bu_config.bin` file has a size set to 0. Then the chip just doesn't reply, either with success or failure, and gets stuck in a failed mode and refuses to respond to anything else. So perhaps that issue would be fixed by giving the code the once-over necessary to support HCI over serial.

Edit: re `load` over serial not working, the issue is perhaps due to using a fixed size [`RTK_FRAGMENT_LENGTH`](https://github.com/google/bumble/blob/main/bumble/drivers/rtk.py#L62), which I suspect it doesn't like over serial. I couldn't immediately figure out how to fix your OO code, but somehow you need to not be using the fixed-size length [here](https://github.com/google/bumble/blob/870217acb3dd5ca2ae964b0282460ce6197bc99b/bumble/drivers/rtk.py#L194), or else the last packet is almost guaranteed to be padded out to a size other than the size mentioned in the data format itself, which it either doesn't like, or doesn't handle gracefully.