mraa.c: fix uart mraa_pin_mode_test

switch statement was missing a clause for MRAA_PIN_UART meaning it always
returned False

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

src/mraa.c

@@ -260,6 +260,9 @@ mraa_pin_mode_test(int pin, mraa_pinmodes_t mode)
             if (plat->pins[pin].capabilites.aio ==1)
                 return 1;
             break;
+        case MRAA_PIN_UART:
+            if (plat->pins[pin].capabilites.uart == 1)
+                return 1;
         default: break;
     }
     return 0;