USB: serial: xr: fix CSIZE handling

The XR21V141X does not have a 5- or 6-bit mode, but the current
implementation failed to properly restore the old setting when CS5 or
CS6 was requested. Instead an invalid request would be sent to the
device.

Fixes: c2d405aa ("USB: serial: add MaxLinear/Exar USB to Serial driver")
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>
Cc: stable@vger.kernel.org	# 5.12
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/xr_serial.c

@@ -467,6 +467,11 @@ static void xr_set_termios(struct tty_struct *tty,
 		termios->c_cflag &= ~CSIZE;
 		if (old_termios)
 			termios->c_cflag |= old_termios->c_cflag & CSIZE;
+		else
+			termios->c_cflag |= CS8;
+
+		if (C_CSIZE(tty) == CS7)
+			bits |= XR21V141X_UART_DATA_7;
 		else
 			bits |= XR21V141X_UART_DATA_8;
 		break;