STM32H7: increase i2c slave rx limit.

Use uint16_t variables for i2c slave_rx_buffer_size and slave_rx_count
variables. This allows to receive more than 255 bytes in slave mode. The
bytes are received one by one in slave mode so there are no hardware
limitations forcing a 1 byte rx count limit.

Author: Mathieu Othacehe

targets/TARGET_STM/TARGET_STM32H7/objects.h

@@ -120,8 +120,8 @@ struct i2c_s {
     volatile uint8_t pending_slave_tx_master_rx;
     volatile uint8_t pending_slave_rx_maxter_tx;
     uint8_t *slave_rx_buffer;
-    volatile uint8_t slave_rx_buffer_size;
-    volatile uint8_t slave_rx_count;
+    volatile uint16_t slave_rx_buffer_size;
+    volatile uint16_t slave_rx_count;
 #endif
 #if DEVICE_I2C_ASYNCH
     uint32_t address;