arm: fix compiler warnings

There were unused variables, incorrect pointer operations
and plan broken string comparison.

Now there's only one - for unused uart3_enabled in beaglebone.c,
but we want to keep it for declaration consistency.

Also fixes #757.

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

src/arm/am335x.c

@@ -50,7 +50,7 @@ mraa_am335x_mmap_write(mraa_gpio_context dev, int value)
     return MRAA_SUCCESS;
 }
 
-static mraa_result_t
+mraa_result_t
 mraa_am335x_mmap_unsetup()
 {
     if (mmap_gpio[0] == NULL) {