uart_line_ctrl_set Function not implemented

Hi,

I'm using the bare hci_uart example and try to rise the RTS line manually. Therefore I use the API

int err = uart_line_ctrl_set(hci_uart_dev, UART_LINE_CTRL_RTS, 1);
and added 
CONFIG_UART_LINE_CTRL=y
in proj.conf. The resulting error code is ENOSYS (88), "Function not implemented". Am I missing some more config values? 
Regards,
Oliver
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