signed cast to avoid compilation warning

src/Nano33BLEflash.cpp

@@ -67,7 +67,7 @@ int flashErasePage(int page)
     }
     return -3;
   }
-  if (page >= NRF_FICR->CODESIZE) {  // 256
+  if (page >= (signed)(NRF_FICR->CODESIZE)) {  // 256
     if (Serial){
       Serial.println("Attempt to erase non-existant page denied");
       Serial.println("Advice:  use pages b/t 50 and 255");