Realtek firmware manual load not working on RTL8761BU on Linux

I believe the fact that Linux aggressively loads its own firmware onto a plugged in RTL8761BU-based USB dongle is perhaps obscuring the fact that AFAICT the load of firmware via Bumble isn't working.

## Reproduction:

Performed on Ubuntu 24.04.

Disable firmware loading in Linux by moving the files out of the way:

```
(venv) a@b:~/Downloads/bumble$ sudo mv /lib/firmware/rtl_bt/rtl8761bu_fw.bin.zst /lib/firmware/rtl_bt/rtl8761bu_fw.bin.zst.backup
(venv) a@b:~/Downloads/bumble$ sudo mv /lib/firmware/rtl_bt/rtl8761bu_config.bin.zst /lib/firmware/rtl_bt/rtl8761bu_config.bin.zst.backup
```

Power dongle off and on again, and Linux will fail to load the firmware on to it. From `dmesg`

```
[ 8107.804803] Bluetooth: hci0: RTL: examining hci_ver=0a hci_rev=dfc6 lmp_ver=0a lmp_subver=d922
[ 8108.010940] Bluetooth: hci0: RTL: examining hci_ver=0a hci_rev=000b lmp_ver=0a lmp_subver=8761
[ 8108.011919] Bluetooth: hci0: RTL: rom_version status=0 version=1
[ 8108.011926] Bluetooth: hci0: RTL: loading rtl_bt/rtl8761bu_fw.bin
[ 8108.011988] bluetooth hci0: Direct firmware load for rtl_bt/rtl8761bu_fw.bin failed with error -2
[ 8108.011994] Bluetooth: hci0: RTL: firmware file rtl_bt/rtl8761bu_fw not found
```

Then try to load the firmware with bumble on the latest git commit:

```
(venv) a@b:~/Downloads/bumble$ git log -1
commit a275c399a3c2d321291e30fed5ff619dce124936 (HEAD -> main, origin/main, origin/HEAD)
Merge: c98275f 0b19347
Author: zxzxwu <92432172+zxzxwu@users.noreply.github.com>
Date: Thu Aug 7 16:52:17 2025 +0800

Merge pull request #734 from khsiao-google/le_subrating

Support LE Subrating
(venv) a@b:~/Downloads/bumble$ bumble-rtk-util parse ~/.local/share/bumble/firmware/realtek/rtl8761bu_fw.bin
Firmware: version=0xDFC6D922 project_id=0x000E
Patch: chip_id=0x0001, 14048 bytes, SVN Version=00005060
Patch: chip_id=0x0002, 30204 bytes, SVN Version=D6D525A4
(venv) a@b:~/Downloads/bumble$ bumble-rtk-util info usb:2550:8761
Driver:
ROM: 8761
Firmware: rtl8761bu_fw.bin
Config: rtl8761bu_config.bin

(venv) a@b:~/Downloads/bumble$ bumble-rtk-util load usb:2550:8761
(venv) a@b:~/Downloads/bumble$ bumble-rtk-util info usb:2550:8761
Driver:
ROM: 8761
Firmware: rtl8761bu_fw.bin
Config: rtl8761bu_config.bin
```

If the firmware load were working, I believe the ROM version should be showing up as the patched version of 0xD922 (like it does when one lets Linux load the patches.)