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>

drivers/spi/atmel_spi.c

@@ -136,13 +136,11 @@ int spi_xfer(struct spi_slave *slave, unsigned int bitlen,
 	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;