usb: cdc: Semaphore per CDC_ACM instance.

Usage of `pool_wait_sem` was incorrect. The semaphore shall be
dependent on context it was called for. In case of many CDC_ACM
devices one could possibly block others. This fixes an issue by
making semaphore a part of the internal CDC_ACM device structure.

Signed-off-by: Emil Obalski <emil.obalski@nordicsemi.no>