Bluetooth: btmtksdio: Fix kernel oops in btmtksdio_interrupt

In the Linux kernel, the following vulnerability has been resolved:

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.