usb: device_next: align CDC ACM UART with Interrupt-driven UART API

Align CDC ACM UART with Interrupt-driven UART API behavior description.
Use the same flags in uart_irq_rx_ready(), uart_irq_tx_ready(), and
uart_irq_is_pending(), which are updated after each uart_irq_update()
call. Allow TX FIFO to be filled if there is space.

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