staging: most: destroy cdev when channel gets disconnected

When a channel is being removed while an application holds the
corresponding character device, this device is going to be destroyed only
after the application closes the file descriptor and releases character
device. In case the channel appears again before the application closes the
file descriptor it holds, the channel cannot be linked.

This patch changes the described behavior and destroys the character
device at the time the channel get disconnected from the AIM.

Signed-off-by: Andrey Shvetsov <andrey.shvetsov@k2l.de>
Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>