More warning cleanup

- physically suppress warnings in TinyUSB headers using pragmas so they don't break -Werror compilation of external files that include them
 - fix compiler warnings in rp2040 port
 - add cmake method to rp2040 port to allow an external project to suppress warnings in TinyUSB itself

src/device/dcd.h

@@ -106,7 +106,14 @@ typedef struct TU_ATTR_ALIGNED(4)
 void dcd_init       (uint8_t rhport);
 
 // Interrupt Handler
+#if __GNUC__
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wredundant-decls"
+#endif
 void dcd_int_handler(uint8_t rhport);
+#if __GNUC__
+#pragma GCC diagnostic pop
+#endif
 
 // Enable device interrupt
 void dcd_int_enable (uint8_t rhport);