MT7902 Bluetooth (0489:e156) not supported - btusb/btmtk

Reporting this rather than submitting a patch. Happy for a maintainer to
pick these up.

Hardware: Minisforum UM880 Plus, MediaTek combo card
          (BT reports hardware variant 0x7902; Wi-Fi binds mt7921e)
USB ID: 0489:e156 (Foxconn/Hon Hai)
Kernel: 7.1.8 (also tested 6.18.44-lts)

Symptom without patch:
  Bluetooth: hci0: Opcode 0x0c03 failed: -110
  No controller appears; Wi-Fi (mt7921e) works fine.

Two issues found:

1. drivers/bluetooth/btusb.c
   USB ID 0489:e156 is missing from the device table. Neighbouring
   IDs (0xe153, 0xe158) are present with BTUSB_MEDIATEK. Without the
   entry, btusb binds via the generic vendor/interface rule but
   without BTUSB_MEDIATEK, so btmtk setup never runs.

   Adding:
     { USB_DEVICE(0x0489, 0xe156), .driver_info = BTUSB_MEDIATEK |
                                                  BTUSB_WIDEBAND_SPEECH },

   changes the error to:
     Bluetooth: hci0: Unsupported hardware variant (00007902)

2. drivers/bluetooth/btmtk.c
   Hardware variant 0x7902 is not handled in the setup switch.
   Firmware BT_RAM_CODE_MT7902_1_1_hdr.bin already ships in
   linux-firmware, and btmtk_fw_get_filename() generates the correct
   path via the default branch.

   Adding `case 0x7902:` alongside 0x7922/0x7925/0x7961 results in:
     Bluetooth: hci0: HW/SW Version: 0x008a008a, Build Time: 20250826211444
     Bluetooth: hci0: Device setup in 2312557 usecs

   Controller then works normally; DualSense pairs and functions.
   Grouping 0x7902 with the existing 79xx case works on my hardware,
   but a maintainer may want to confirm this is the correct handling.

Both changes tested on 6.18.44-lts, 7.1.8-arch1 and 7.1.8-zen via DKMS.

Kind Regards,
Kaiser Bh