stm32/pyb_i2c: Fix pyb.I2C to work with dma=True on F4 MCUs.

Prior to this commit, excuting this code:

    i2c = I2C(1, I2C.CONTROLLER, dma=True)
    i2c.send(data, addr=i2c_addr)

the call to i2c.send() does not return and the board needs a reset.  This
code works when dma=False.

According to the specification, I2Cx_EV_IRQHandler should:
- Write DR to address when Start condition generated.
- Clear ADDR by reading SR2 after reading SR2 when address sent.

These processes are included in HAL_I2C_EV_IRQHandler(), however the
firmware size increses about 2KB if HAL_I2C_EV_IRQHandler is called.  This
commit adds above processes to i2c_ev_irq_handler, and increases firmware
by less than 100 bytes.

Fixes issue #2643.

ports/stm32/pyb_i2c.c

@@ -472,7 +472,17 @@ void i2c_ev_irq_handler(mp_uint_t i2c_id) {
 
     #if defined(STM32F4)
 
-    if (hi2c->Instance->SR1 & I2C_FLAG_BTF && hi2c->State == HAL_I2C_STATE_BUSY_TX) {
+    if (hi2c->Instance->SR1 & I2C_FLAG_SB) {
+        if (hi2c->State == HAL_I2C_STATE_BUSY_TX) {
+            hi2c->Instance->DR = I2C_7BIT_ADD_WRITE(hi2c->Devaddress);
+        } else {
+            hi2c->Instance->DR = I2C_7BIT_ADD_READ(hi2c->Devaddress);
+        }
+    } else if (hi2c->Instance->SR1 & I2C_FLAG_ADDR) {
+        __IO uint32_t tmp_sr2;
+        tmp_sr2 = hi2c->Instance->SR2;
+        UNUSED(tmp_sr2);
+    } else if (hi2c->Instance->SR1 & I2C_FLAG_BTF && hi2c->State == HAL_I2C_STATE_BUSY_TX) {
         if (hi2c->XferCount != 0U) {
             hi2c->Instance->DR = *hi2c->pBuffPtr++;
             hi2c->XferCount--;