mraa: Fix compiler warnings unused vars and useless statements

Should have 0 functionality effect, just cleans up a few things. Fixes #577

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

examples/cycle-pwm3.c

@@ -49,6 +49,7 @@ main()
             value = 0.0f;
         }
         float output = mraa_pwm_read(pwm);
+        printf("PWM value is %f\n", output);
     }
     //! [Interesting]
     return 0;

examples/iio_driver.c

@@ -55,7 +55,7 @@ interrupt(char* data)
     mraa_iio_channel* channels = mraa_iio_get_channels(iio_device0);
     int i = 0;
 
-    for (i; i < mraa_iio_get_channel_count(iio_device0); i++) {
+    for (; i < mraa_iio_get_channel_count(iio_device0); i++) {
         if (channels[i].enabled) {
             printf("channel %d - bytes %d\n", channels[i].index, channels[i].bytes);
             switch (channels[i].bytes) {

examples/mraa-i2c.c

@@ -144,7 +144,6 @@ i2c_set_exit:
 void
 i2c_detect_devices(int bus)
 {
-    mraa_result_t status = MRAA_SUCCESS;
     mraa_i2c_context i2c = mraa_i2c_init(bus);
     if (i2c == NULL) {
         return;

examples/spi_mcp4261.c

@@ -33,7 +33,6 @@ main(int argc, char** argv)
     //! [Interesting]
     mraa_spi_context spi;
     spi = mraa_spi_init(0);
-    unsigned int response = 0;
     printf("Hello, SPI initialised\n");
     uint8_t data[] = { 0x00, 100 };
     uint8_t* recv;