Driver i2c: improve frequency setting function

Author: maciejbocianski

drivers/I2C.cpp

@@ -18,6 +18,7 @@
 #include "drivers/I2C.h"
 #include "drivers/DigitalInOut.h"
 #include "platform/mbed_wait_api.h"
+#include "platform/mbed_assert.h"
 
 #if DEVICE_I2C
 
@@ -58,6 +59,7 @@ I2C::~I2C()
 void I2C::frequency(int hz)
 {
     lock();
+    MBED_ASSERT(_hz > 0);
     _hz = (uint32_t)hz;
 
     // We want to update the frequency even if we are already the bus owners

drivers/I2CSlave.cpp

@@ -14,6 +14,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+#include "platform/mbed_assert.h"
 #include "drivers/I2CSlave.h"
 
 #if DEVICE_I2CSLAVE
@@ -33,7 +34,8 @@ I2CSlave::~I2CSlave()
 
 void I2CSlave::frequency(int hz)
 {
-    i2c_frequency(&_i2c, hz);
+    MBED_ASSERT(hz > 0);
+    i2c_frequency(&_i2c, (uint32_t)hz);
 }
 
 void I2CSlave::timeout(uint32_t timeout)