Narrowly silence new (GCC 8.1+) warning

[henrygab]

Fixes #287

The warnings look like:

      Line 338 Char 37
      warning: 'void* memcpy(void*, const void*, size_t)' 
               writing to an object of type 'struct DmacDescriptor'
               with no trivial copy-assignment [-Wclass-memaccess]

[hathach]

The above solution does not work due to trivial class initialization issue, I have come to using the good-old void* that could suppress the warning

            memcpy((void*) &extraReadDescriptors[i], firstReadDescriptor,
              sizeof(DmacDescriptor));