mraa.c: make mraa_gpio_lookup() return proper pin index

Pinmap field value is an OS pin number, not mraa pin index,
so make the function return the proper one.

Fixes #817.

Signed-off-by: Alex Tereschenko <alext.mkrs@gmail.com>

src/mraa.c

@@ -883,7 +883,7 @@ mraa_gpio_lookup(const char* pin_name)
     for (i = 0; i < plat->gpio_count; i++) {
          if (plat->pins[i].name != NULL &&
              strncmp(pin_name, plat->pins[i].name, strlen(plat->pins[i].name) + 1) == 0) {
-             return plat->pins[i].gpio.pinmap;
+             return i;
          }
     }
     return -1;