Bluetooth: btmtksdio: Check function enabled before doing close

Check BTMTKSDIO_FUNC_ENABLED flag before doing close to prevent
btmtksdio_close been called twice.

Fixes: 6ac4233a ("Bluetooth: btmtksdio: Prevent enabling interrupts after IRQ handler removal")
Signed-off-by: Chris Lu <chris.lu@mediatek.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>