I needed to use DS3231, I remember that in past it worked ok, but now for...

I needed to use DS3231, I remember that in past it worked ok, but now for stm32f4xx is used another driver (chip specific, stm32f40xxx_i2c.c) and DS3231 driver doesn't work. After investigating a problem I found that I2C driver (isr routine) has a few places there it sends stop bit even if not all messages are managed. So, e.g., removing stm32_i2c_sendstop (#1744) and adding stm32_i2c_sendstart after data reading helps to make DS3231 working.  Verified by David Sidrane.