The function hso_serial_common_free() is called either by the cleanup
method of the tty or by the usb disconnect method.
In the former case, the usb_disconnect() has been already called
and the sysfs group associated to the device has been removed.
By calling tty_unregister directly from the usb_disconnect() method,
we avoid a warning due to the removal of the sysfs group of the usb
device.

Example of warning:
------------[ cut here ]------------
WARNING: CPU: 0 PID: 778 at fs/sysfs/group.c:225 sysfs_remove_group+0x50/0x94()
sysfs group c0645a88 not found for kobject 'ttyHS5'
Modules linked in:
CPU: 0 PID: 778 Comm: kworker/0:3 Tainted: G        W      3.18.0+ #105
Workqueue: events release_one_tty
[<c000dfe4>] (unwind_backtrace) from [<c000c014>] (show_stack+0x14/0x1c)
[<c000c014>] (show_stack) from [<c0016bac>] (warn_slowpath_common+0x5c/0x7c)
[<c0016bac>] (warn_slowpath_common) from [<c0016c60>] (warn_slowpath_fmt+0x30/0x40)
[<c0016c60>] (warn_slowpath_fmt) from [<c00ddd14>] (sysfs_remove_group+0x50/0x94)
[<c00ddd14>] (sysfs_remove_group) from [<c0221e44>] (device_del+0x30/0x190)
[<c0221e44>] (device_del) from [<c0221fb0>] (device_unregister+0xc/0x18)
[<c0221fb0>] (device_unregister) from [<c0221fec>] (device_destroy+0x30/0x3c)
[<c0221fec>] (device_destroy) from [<c01fe1dc>] (tty_unregister_device+0x2c/0x5c)
[<c01fe1dc>] (tty_unregister_device) from [<c029a428>] (hso_serial_common_free+0x2c/0x88)
[<c029a428>] (hso_serial_common_free) from [<c029a4c0>] (hso_serial_ref_free+0x3c/0xb8)
[<c029a4c0>] (hso_serial_ref_free) from [<c01ff430>] (release_one_tty+0x30/0x84)
[<c01ff430>] (release_one_tty) from [<c00271d4>] (process_one_work+0x21c/0x3c8)
[<c00271d4>] (process_one_work) from [<c0027758>] (worker_thread+0x3d8/0x560)
[<c0027758>] (worker_thread) from [<c002be4c>] (kthread+0xc0/0xcc)
[<c002be4c>] (kthread) from [<c0009630>] (ret_from_fork+0x14/0x24)
---[ end trace cb88537fdc8fa208 ]---
Signed-off-by: Olivier Sobrie <olivier@sobrie.be>
Signed-off-by: David S. Miller <davem@davemloft.net>