HID: steelseries: Add async status interface support

Some headsets expose a second HID interface that sends battery and
connection updates on its own. Watching that interface lets the driver
stop polling the sync interface. Add a
steelseries_device_info::async_interface field and the code to handle
it:

  - The driver binds both the sync and async interfaces. The async
    interface shares the steelseries_device created by the sync
    interface. It finds the sibling with usb_ifnum_to_if(), and before
    trusting its intfdata it rejects non-HID siblings by descriptor
    class and holds the sibling's device lock across the lookup, so a
    crafted device cannot cause a type-confused read and a concurrent
    unbind cannot free the hid_device from under it. It then takes a
    reference and returns -EPROBE_DEFER until the sync interface has
    probed. If the sync interface never binds, the async interface
    defers forever, which is fine here.
  - raw_event() now holds sd->lock and re-checks sd->removed so events
    on either interface are serialised against removal.
  - status_work runs once for async devices instead of rearming. A
    single status request is sent when the headset connects to get the
    initial battery level.

No device sets async_interface yet. This is the infrastructure for the
next commit.

Signed-off-by: Sriman Achanta <srimanachanta@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>

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