stm32/i2c: Fix num_acks calculation in i2c_write for F0 and F7 MCU's.

Due to buffering of outgoing bytes on the I2C bus, detection of a NACK
using the ISR_NACKF flag needs to account for the case where ISR_NACKF
corresponds to the previous-to-previous byte.

ports/stm32/i2c.c

@@ -421,8 +421,13 @@ int i2c_write(i2c_t *i2c, const uint8_t *src, size_t len, size_t next_len) {
         if ((ret = i2c_wait_isr_set(i2c, I2C_ISR_TCR | I2C_ISR_TC | I2C_ISR_STOPF))) {
             return ret;
         }
-        if (i2c->ISR & I2C_ISR_NACKF) {
+        uint32_t isr = i2c->ISR;
+        if (isr & I2C_ISR_NACKF) {
             // Slave did not respond to byte so stop sending
+            if (!(isr & I2C_ISR_TXE)) {
+                // The TXDR is still full so the byte previous to that wasn't actually ACK'd
+                --num_acks;
+            }
             break;
         }
         ++num_acks;