hwmon: (powerz) Fix use-after-free on USB disconnect

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

hwmon: (powerz) Fix use-after-free on USB disconnect

After powerz_disconnect() frees the URB and releases the mutex, a

subsequent powerz_read() call can acquire the mutex and call

powerz_read_data(), which dereferences the freed URB pointer.

Fix by:

- Setting priv->urb to NULL in powerz_disconnect() so that

powerz_read_data() can detect the disconnected state.

- Adding a !priv->urb check at the start of powerz_read_data()

to return -ENODEV on a disconnected device.

- Moving usb_set_intfdata() before hwmon registration so the

disconnect handler can always find the priv pointer.