AP_HAL_ChibiOS: adjust GPIO settings for serial passthru

libraries/AP_HAL_ChibiOS/RCOutput.cpp

@@ -1819,8 +1819,10 @@ uint16_t RCOutput::serial_read_bytes(uint8_t *buf, uint16_t len)
     }
     pwm_group &group = *serial_group;
     const ioline_t line = group.pal_lines[group.serial.chan];
-    uint32_t gpio_mode = PAL_MODE_INPUT_PULLUP;
-    uint32_t restore_mode = PAL_MODE_ALTERNATE(group.alt_functions[group.serial.chan]) | PAL_STM32_OSPEED_MID2 | PAL_STM32_OTYPE_PUSHPULL;
+    // keep speed low to avoid noise when switching between input and output
+    uint32_t gpio_mode = PAL_STM32_MODE_INPUT | PAL_STM32_OTYPE_PUSHPULL | PAL_STM32_PUPDR_PULLUP | PAL_STM32_OSPEED_LOWEST;
+    // restore the line to what it was before
+    iomode_t restore_mode = palReadLineMode(line);
     uint16_t i = 0;
 
 #if RCOU_SERIAL_TIMING_DEBUG