drivers: i2c_ll_stm32_v1: clear stop bit befor setting start bit

Sometimes the stop bit is still set when starting the next transaction.
When that happens the hardware will generate a start directly followed
by a stop. This will not be detected by the driver and it will endlessly
wait for the next interrupt that will never come.

Signed-off-by: Erwin Rol <erwin@erwinrol.com>