STM32: Fix I2C stop condition

Need to ensure the transmission has been started before sending a STOP condition.

Issue found on the NUCLEO_H743ZI due certainly to the high-speed clock used.

But this is normally needed also on all STM32 devices using the I2C peripheral version 2.

Author: bcostm

targets/TARGET_STM/i2c_api.c

@@ -590,6 +590,17 @@ int i2c_stop(i2c_t *obj)
 #endif
     // Disable reload mode
     handle->Instance->CR2 &= (uint32_t)~I2C_CR2_RELOAD;
+    
+    // Ensure the transmission is started before sending a stop
+    if ((handle->Instance->CR2 & (uint32_t)I2C_CR2_RD_WRN) == 0) {
+      timeout = FLAG_TIMEOUT;
+      while (!__HAL_I2C_GET_FLAG(handle, I2C_FLAG_TXIS)) {
+          if ((timeout--) == 0) {
+              return I2C_ERROR_BUS_BUSY;
+          }
+      }
+    }
+    
     // Generate the STOP condition
     handle->Instance->CR2 |= I2C_CR2_STOP;