Bluetooth: btbcm: fix missing of_node_put() in btbcm_get_board_name()

of_find_node_by_path() returns a pointer to a device_node with its
refcount incremented, and a call to of_node_put() is required to
decrement the refcount again and avoid leaking the resource.

If 'of_property_read_string_index(root, "compatible", 0, &tmp)' fails,
the function returns without calling of_node_put(root) before doing so.

The automatic cleanup attribute can be used by means of the __free()
macro to automatically call of_node_put() when the variable goes out of
scope, fixing the issue and also accounting for new error paths.

Fixes: 63fac334 ("Bluetooth: btbcm: Support per-board firmware variants")
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>