drivers: i2c: stm32: Fix STM32 I2C hang on read

ensure LL_I2C_TransmitData8() is always called when
TXIS is set, even if read_processed() returns an error.
This prevents the peripheral from clock-stretching
indefinitely and blocking the I2C bus when the master
reads from the slave.

Signed-off-by: Khaoula Bidani <khaoula.bidani-ext@st.com>