Bluetooth: btmtk: Mark all stub functions as inline

Several recent patches added static stubs to btmtk.h without the inline
keyword, which causes instances of -Wunused-function when those stubs
are not used anywhere in a file that includes the header:

  In file included from drivers/bluetooth/btusb.c:28:
  drivers/bluetooth/btmtk.h:254:13: warning: 'btmtk_fw_get_filename' defined but not used [-Wunused-function]
    254 | static void btmtk_fw_get_filename(char *buf, size_t size, u32 dev_id,
        |             ^~~~~~~~~~~~~~~~~~~~~
  drivers/bluetooth/btmtk.h:249:12: warning: 'btmtk_process_coredump' defined but not used [-Wunused-function]
    249 | static int btmtk_process_coredump(struct hci_dev *hdev, struct sk_buff *skb)
        |            ^~~~~~~~~~~~~~~~~~~~~~
  drivers/bluetooth/btmtk.h:243:12: warning: 'btmtk_register_coredump' defined but not used [-Wunused-function]
    243 | static int btmtk_register_coredump(struct hci_dev *hdev, const char *name,
        |            ^~~~~~~~~~~~~~~~~~~~~~~
  drivers/bluetooth/btmtk.h:233:12: warning: 'btmtk_setup_firmware' defined but not used [-Wunused-function]
    233 | static int btmtk_setup_firmware(struct hci_dev *hdev, const char *fwname,
        |            ^~~~~~~~~~~~~~~~~~~~
  drivers/bluetooth/btmtk.h:227:12: warning: 'btmtk_setup_firmware_79xx' defined but not used [-Wunused-function]
    227 | static int btmtk_setup_firmware_79xx(struct hci_dev *hdev, const char *fwname,
        |            ^~~~~~~~~~~~~~~~~~~~~~~~~

Add inline to all the stubs in btmtk.h (even ones that do not currently
have any warnings associated with them) to ensure there are never unused
function warnings from these stubs, as is customary for the kernel.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>