drivers: i2c_stm32_v2: fix assert in i2c event isr

For systems where the CPU / NVIC bus are considerably
faster than the peripheral bus used for I2C, there is
a risk that the isr handler returns before I2C controller
had time to clear its interrupt status flags

This would cause immediate re-entry to isr handler
which begins read from ISR register.
During time it take to re-enter and read from ISR register,
flags had time to be cleared
The isr handler expects that a flag is set, and in this case
when its not, an assert is triggered

The solution here is to make a dummy read from the ISR register before
returning from isr handler, this forces CPU to wait as long as needed
to ensure all writes to the I2C peripheral are completed

Fixes issue #97904

Signed-off-by: Erik Andersson <[email protected]>

Author: erian747

drivers/i2c/i2c_ll_stm32_v2.c

@@ -578,6 +578,11 @@ void i2c_stm32_event(const struct device *dev)
 		__ASSERT_NO_MSG(0);
 	}
 
+	/* Make a dummy read from ISR to ensure we don't return before
+	 * i2c controller had a chance to clear its interrupt flags due
+	 * to bus delays
+	 */
+	(void)LL_I2C_ReadReg(regs, ISR);
 	return;
 
 irq_xfer_completed: