Fix compiler warning: comparison of unsigned expression

Fix #44
cores/arduino/stm32/PinNamesTypes.h:132:44: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
 #define STM_VALID_PINNAME(X) ((STM_PORT(X) >= FirstPort) && (STM_PORT(X) <= LastPort))

Signed-off-by: Frederic.Pillon <frederic.pillon@st.com>

Author: fpistm

cores/arduino/stm32/PinNamesTypes.h

@@ -129,7 +129,10 @@ typedef enum {
 #define STM_PORT(X) (((uint32_t)(X) >> 4) & 0xF)
 #define STM_PIN(X)  ((uint32_t)(X) & 0xF)
 // Check PinName is valid: FirstPort <= PortName <= LastPort
-#define STM_VALID_PINNAME(X) ((STM_PORT(X) >= FirstPort) && (STM_PORT(X) <= LastPort))
+// As FirstPort is equal to 0 and STM_PORT cast as an unsigned
+// (STM_PORT(X) >= FirstPort)  is always true
+//#define STM_VALID_PINNAME(X) ((STM_PORT(X) >= FirstPort) && (STM_PORT(X) <= LastPort))
+#define STM_VALID_PINNAME(X) (STM_PORT(X) <= LastPort)
 
 #define STM_GPIO_PIN(X) ((uint16_t)(1<<STM_PIN(X)))
 /*  Defines to be used by application */