Bluetooth: btmtksdio: handle runtime pm only when sdio_func is available

Runtime pm ops is not aware the sdio_func status that is probably
being disabled by btmtksdio_close. Thus, we are only able to access the
sdio_func for the runtime pm operations only when the sdio_func is
available.

Fixes: 7f3c563c

 ("Bluetooth: btmtksdio: Add runtime PM support to SDIO based Bluetooth")
Co-developed-by: Mark-yw Chen <mark-yw.chen@mediatek.com>
Signed-off-by: Mark-yw Chen <mark-yw.chen@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>