usb: cdc_acm: Do not change rx/tx_irq_ena flags on RESET and CONFIGURED

Both tx_irq_ena and rx_irq_ena flags shall be updated only
by using UART API. Those flags are used to let the device
start sending or receiving data. Changing those flags without
awareness of the user may lead to errors.

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