u-boot: v2026.04: helios64: preserve SPI I/O error in read_otp_data

spi_write_then_read() returned value was only used to set has_been_read,
then header/CRC checks ran against whatever ended up in the otp struct.
On a real SPI I/O failure, the struct could contain partial garbage,
header/CRC would fail, and the caller got a generic -1 — losing the
original errno and conflating I/O errors with invalid OTP data.

Handle the I/O error path explicitly:
- On ret != 0: log, keep has_been_read = 0 (so the next call can retry),
  and return the original ret.
- On ret == 0: set has_been_read = 1 and proceed to header/CRC validation
  as before.

Addresses a CodeRabbit review comment on PR armbian#9675.

Author: iav

patch/u-boot/v2026.04/board_helios64/board/sys_otp.c

@@ -177,7 +177,13 @@ int read_otp_data(void)
 	show_otp_data();
 #endif
 
-	has_been_read = (ret == 0) ? 1 : 0;
+	if (ret) {
+		debug("SPI: Failed to read OTP: %d\n", ret);
+		/* Leave has_been_read = 0 so a later call can retry. */
+		return ret;
+	}
+	has_been_read = 1;
+
 	if (!is_valid_header())
 		goto data_invalid;