Hardware flow control for serial requires the usage of DMAC channels which is not yet supported for SAMv7 MCU. However the same config option CONFIG_SERIAL_IFLOWCONTROL is also used for USB CDC/ACM flow control which works well. Therefore the warning message should be raised only if flow control is configured for USART driver and not for USB CDC/ACM. Signed-off-by: Michal Lenc <michallenc@seznam.cz>