update rp2040 warnings

- remove "-Wno-stringop-overflow -Wno-array-bounds"
- skip -Wconversion for gcc 9 and prior
- suppress_tinyusb_warnings only when building with gcc 9 and below

examples/device/cdc_dual_ports/src/main.c

@@ -55,17 +55,19 @@ int main(void)
 // with Serial0 as all lower case, Serial1 as all upper case
 static void echo_serial_port(uint8_t itf, uint8_t buf[], uint32_t count)
 {
+  uint8_t const case_diff = 'a' - 'A';
+
   for(uint32_t i=0; i<count; i++)
   {
     if (itf == 0)
     {
       // echo back 1st port as lower case
-      if (isupper(buf[i])) buf[i] = (uint8_t) (buf[i] + 'a' - 'A');
+      if (isupper(buf[i])) buf[i] += case_diff;
     }
     else
     {
       // echo back 2nd port as upper case
-      if (islower(buf[i])) buf[i] = (uint8_t) (buf[i] - 'a' - 'A');
+      if (islower(buf[i])) buf[i] -= case_diff;
     }
 
     tud_cdc_n_write_char(itf, buf[i]);