ftp: zero the bytes counter only on ftp client start

modules/ftp.c

@@ -686,8 +686,6 @@ static unsigned data_sink(void *arg, const char* ptr, unsigned len)
   } else {
     printf("ftp: the firmware is downloaded, verifying...\r\n");
     uint32_t fw_size = received_bytes_count;
-    // next time it comes, overwrite the existing SPI contents
-    received_bytes_count = 0;
 
     bool good_firmware = validate_spif_firmware(fw_size);
     if (good_firmware) {