[ot] hw/opentitan: ot_spi_device: change end of JEDEC ID buffer to 0s

Expected by spi_passthru_test, see:
https://github.com/lowRISC/opentitan/blob/master/sw/host/tests/chip/spi_device/src/spi_passthru.rs#L70-L80

Signed-off-by: Alice Ziuziakowska <[email protected]>

Author: ziuziakowska

hw/opentitan/ot_spi_device.c

@@ -995,8 +995,9 @@ static void ot_spi_device_flash_decode_read_jedec(OtSPIDeviceState *s)
      */
     f->buffer[f->len++] = (uint8_t)(jedec_device >> 0u);
     f->buffer[f->len++] = (uint8_t)(jedec_device >> 8u);
-    memset(&f->buffer[f->len], (int)SPI_DEFAULT_TX_VALUE,
-           SPI_FLASH_BUFFER_SIZE - f->len);
+    /* after the end of JEDEC ID is 0s on OpenTitan */
+    memset(&f->buffer[f->len], (int)0u, SPI_FLASH_BUFFER_SIZE - f->len);
+    f->len = SPI_FLASH_BUFFER_SIZE;
     f->src = f->buffer;
     FLASH_CHANGE_STATE(s, BUFFER);
 }