AT91: Correct configuration define value

The MTD nand driver for Atmel chips is atmel_nand and not at91_nand anymore.
Change wrong configuration variables that were remaining.

Reported-by: Christoph Egger <siccegge@cs.fau.de>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
diff --git a/arch/arm/mach-at91/board-at572d940hf_ek.c b/arch/arm/mach-at91/board-at572d940hf_ek.c
index 5daff27..4665162 100644
--- a/arch/arm/mach-at91/board-at572d940hf_ek.c
+++ b/arch/arm/mach-at91/board-at572d940hf_ek.c
@@ -216,7 +216,7 @@
 /*	.rdy_pin	= AT91_PIN_PC16, */
 	.enable_pin	= AT91_PIN_PA15,
 	.partition_info	= nand_partitions,
-#if defined(CONFIG_MTD_NAND_AT91_BUSWIDTH_16)
+#if defined(CONFIG_MTD_NAND_ATMEL_BUSWIDTH_16)
 	.bus_width_16	= 1,
 #else
 	.bus_width_16	= 0,