usb: device: cdc_acm: send more than 1 byte in poll out

Schedule CDC ACM tx work to happen 1 ms after first unprocessed data
write. This gives enough leeway for the task writing to the queue to
write more than 1 byte before USB workqueue preempts and schedules IN
transfer. Sending more than 1 byte at a time increases data throughput
because transaction overhead remains the same regardless of packet size.

Prior to this change, virtually every IN transaction carried only one
character when CDC ACM was used as a console.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>