Bluetooth: btmtksdio: Fix kernel oops in btmtksdio_interrupt

Fix the following kernel oops in btmtksdio_interrrupt

[   14.339134]  btmtksdio_interrupt+0x28/0x54
[   14.339139]  process_sdio_pending_irqs+0x68/0x1a0
[   14.339144]  sdio_irq_work+0x40/0x70
[   14.339154]  process_one_work+0x184/0x39c
[   14.339160]  worker_thread+0x228/0x3e8
[   14.339168]  kthread+0x148/0x3ac
[   14.339176]  ret_from_fork+0x10/0x30

That happened because hdev->power_on is already called before
sdio_set_drvdata which btmtksdio_interrupt handler relies on is not
properly set up.

The details are shown as the below: hci_register_dev would run
queue_work(hdev->req_workqueue, &hdev->power_on) as WQ_HIGHPRI
workqueue_struct to complete the power-on sequeunce and thus hci_power_on
may run before sdio_set_drvdata is done in btmtksdio_probe.

The hci_dev_do_open in hci_power_on would initialize the device and enable
the interrupt and thus it is possible that btmtksdio_interrupt is being
called right before sdio_set_drvdata is filled out.

When btmtksdio_interrupt is being called and sdio_set_drvdata is not filled
, the kernel oops is going to happen because btmtksdio_interrupt access an
uninitialized pointer.

Fixes: 9aebfd4a

 ("Bluetooth: mediatek: add support for MediaTek MT7663S and MT7668S SDIO devices")
Reviewed-by: Mark Chen <markyawenchen@gmail.com>
Co-developed-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Yake Yang <yake.yang@mediatek.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>