drivers/spi/atmel_spi.c: Fix GCC 4.6 warning

Fix:
atmel_spi.c: In function 'spi_xfer':
atmel_spi.c:139:7: warning: variable 'ret' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Anatolij Gustschin <agust@denx.de>
diff --git a/drivers/spi/atmel_spi.c b/drivers/spi/atmel_spi.c
index 33e38b6..83ef8e8 100644
--- a/drivers/spi/atmel_spi.c
+++ b/drivers/spi/atmel_spi.c
@@ -136,13 +136,11 @@
 	unsigned int	len_tx;
 	unsigned int	len_rx;
 	unsigned int	len;
-	int		ret;
 	u32		status;
 	const u8	*txp = dout;
 	u8		*rxp = din;
 	u8		value;
 
-	ret = 0;
 	if (bitlen == 0)
 		/* Finish any previously submitted transfers */
 		goto out;