mtd, omap: fix case NAND_OMAP_GPMC_PREFETCH not defined

The patch c316f577b4 breaks
siemens boards because prefetch mode is not enabled.
I assume it breaks other boards as well that don't use
prefetch.

This patch sets read_buf to nand_read_buf if
NAND_OMAP_GPMC_PREFETCH is not defined.

Signed-off-by: Samuel Egli <samuel.egli@siemens.com>
CC: Daniel Mack <zonque@gmail.com>
CC: Guido Martínez <guido@vanguardiasur.com.ar>
CC: Tom Rini <trini@ti.com>
CC: Heiko Schocher <hs@denx.de>

drivers/mtd/nand/omap_gpmc.c

@@ -989,12 +989,15 @@ int board_nand_init(struct nand_chip *nand)
 	if (err)
 		return err;
 
-#ifdef CONFIG_NAND_OMAP_GPMC_PREFETCH
 	/* TODO: Implement for 16-bit bus width */
 	if (nand->options & NAND_BUSWIDTH_16)
 		nand->read_buf = nand_read_buf16;
+#ifdef CONFIG_NAND_OMAP_GPMC_PREFETCH
 	else
 		nand->read_buf = omap_nand_read_prefetch8;
+#else
+	else
+		nand->read_buf = nand_read_buf;
 #endif
 
 	nand->dev_ready = omap_dev_ready;