usb: device_next: use delayable work for TX FIFO in CDC ACM

Use delayable work to reduce CPU load when there is no transfer flow in
the host direction. This also improves the performance of poll out, as
introduced in commit commit fed6bde7
("usb: device: cdc_acm: send more than 1 byte in poll out")
for the legacy CDC ACM implementation.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>