usb: device: cdc_acm: Use ZLP to detect initial host read

Prevent ECHO on Linux by arming IN endpoint with ZLP when interface is
configured and making sure that actual payload is only sent after
initialization timeout. The ZLP is not visible to host side applications
because the applications are really accessing tty buffer and received
ZLP does not modify tty buffer in any way.

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