HAL_ChibiOS: retain OPENDRAIN if set on a pin

this allows OPENDRAIN when set on a pin to be retained when set with a
pinMode(). This fixes a partially lit B/E LED on the Pixhawk4

libraries/AP_HAL_ChibiOS/GPIO.cpp

@@ -17,6 +17,7 @@
 #include "GPIO.h"
 
 #include <AP_BoardConfig/AP_BoardConfig.h>
+#include "hwdef/common/stm32_util.h"
 
 using namespace ChibiOS;
 
@@ -79,6 +80,15 @@ void GPIO::pinMode(uint8_t pin, uint8_t output)
             return;
         }
         g->mode = output?PAL_MODE_OUTPUT_PUSHPULL:PAL_MODE_INPUT;
+#if defined(STM32F7) || defined(STM32H7) || defined(STM32F4)
+        if (g->mode == PAL_MODE_OUTPUT_PUSHPULL) {
+            // retain OPENDRAIN if already set
+            iomode_t old_mode = palReadLineMode(g->pal_line);
+            if ((old_mode & PAL_MODE_OUTPUT_OPENDRAIN) == PAL_MODE_OUTPUT_OPENDRAIN) {
+                g->mode = PAL_MODE_OUTPUT_OPENDRAIN;
+            }
+        }
+#endif
         palSetLineMode(g->pal_line, g->mode);
         g->is_input = !output;
     }

libraries/AP_HAL_ChibiOS/hwdef/fmuv5/hwdef.dat

@@ -290,7 +290,7 @@ PC12 SDMMC_CK SDMMC1
 PD2 SDMMC_CMD SDMMC1
 
 # red LED marked as B/E
-PB1 LED_RED OUTPUT GPIO(90)
+PB1 LED_RED OUTPUT OPENDRAIN GPIO(90)
 
 # green LED marked as PWR. We leave this solid on, but allow
 # for it to be controlled as a relay if needed