Improve handling of CDC ACM line state

- For the CC2538, simplify handling of USB_CDC_ACM_LINE_STATE
  events. Ignore the Carrier Control (RTS) bit when receiving
  a SET_CONTROL_LINE _STATE request, we are a full duplex device.
- Improve behaviour of the CC2531 USB stick when there is no
  process on the host to read IN data. Basically, we adopt the
  CC2538 approach and we only send data when a DTE is present

Closes #196