drivers: i2c: stm32: I2C transaction first transfer must set RESTART flag

The first transfer in an I2C transaction series must always set
the RESTART flag. Since this is not always done by callers,
enforce the flag is set, both in the I2C API and RTIO API entry
points of STM32 I2C RTIO drivers.

Add a sanity test in STM32 I2C v2 RTIO driver on transaction
chunks regarding RESTART and STOP flags, as done on the non-RTIO
driver.


Signed-off-by: Etienne Carriere <etienne.carriere@st.com>