tty: serial: fsl_lpuart: fix the wrong return value in lpuart32_get_mctrl

Patch e60c299 make the lpuart32_get_mctrl always return 0, actually
this will break the functions of device which use flow control such as
Bluetooth.
For lpuart32 plaform, the hardware can handle the CTS automatically.
So we should set TIOCM_CTS active. Also need to set CAR and DSR active.

Patch has been tested on lpuart32 platforms such as imx8qm and imx8ulp.

Fixes: e60c299 ("serial: fsl_lpuart: remove RTSCTS handling from get_mctrl()")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Sherry Sun <sherry.sun@nxp.com>
Link: https://lore.kernel.org/r/20210729083109.31541-1-sherry.sun@nxp.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: SherrySun5

drivers/tty/serial/fsl_lpuart.c

@@ -1415,7 +1415,7 @@ static unsigned int lpuart_get_mctrl(struct uart_port *port)
 
 static unsigned int lpuart32_get_mctrl(struct uart_port *port)
 {
-	unsigned int mctrl = 0;
+	unsigned int mctrl = TIOCM_CAR | TIOCM_DSR | TIOCM_CTS;
 	u32 reg;
 
 	reg = lpuart32_read(port, UARTCTRL);