Fix CardReader::lsDive()

If comparison between signed char(-128 ~ +127) and DIR_NAME_DELETED(uint8_t const 0XE5, it's +229),
it makes incorrect result.

Marlin/cardreader.cpp

       // close() is done automatically by destructor of SdFile
     }
     else {
-      char pn0 = p.name[0];
+      uint8_t pn0 = p.name[0];
       if (pn0 == DIR_NAME_FREE) break;
       if (pn0 == DIR_NAME_DELETED || pn0 == '.') continue;
       if (longFilename[0] == '.') continue;