spi: ep93xx: fix double-free of zeropage on DMA setup failure

If DMA setup fails after allocating the zeropage, the error path frees
the page but leaves espi->zeropage dangling. A subsequent call to
ep93xx_spi_release_dma() sees the non-NULL pointer and frees the page
again.

Clear the pointer after freeing in the error path of
ep93xx_spi_setup_dma().

Fixes: 626a96db11 ("spi/ep93xx: add DMA support")
Signed-off-by: Felix Gu <ustc.gu@gmail.com>
Link: https://patch.msgid.link/20260529-ep93xx-v1-1-9185070ca1fc@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-ep93xx.c

@@ -600,6 +600,7 @@ fail_release_rx:
 	espi->dma_rx = NULL;
 fail_free_page:
 	free_page((unsigned long)espi->zeropage);
+	espi->zeropage = NULL;
 
 	return ret;
 }