Fix MSB check in Xilinx Spartan slave serial mode

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd-electronics.com>
diff --git a/common/spartan2.c b/common/spartan2.c
index ef107bc..2f1ea2c 100644
--- a/common/spartan2.c
+++ b/common/spartan2.c
@@ -441,7 +441,7 @@
 	int ret_val = FPGA_FAIL;	/* assume the worst */
 	Xilinx_Spartan2_Slave_Serial_fns *fn = desc->iface_fns;
 	int i;
-	char  val;
+	unsigned char val;
 
 	PRINTF ("%s: start with interface functions @ 0x%p\n",
 			__FUNCTION__, fn);
diff --git a/common/spartan3.c b/common/spartan3.c
index 56a172e..d329e70 100644
--- a/common/spartan3.c
+++ b/common/spartan3.c
@@ -446,7 +446,7 @@
 	int ret_val = FPGA_FAIL;	/* assume the worst */
 	Xilinx_Spartan3_Slave_Serial_fns *fn = desc->iface_fns;
 	int i;
-	char  val;
+	unsigned char val;
 
 	PRINTF ("%s: start with interface functions @ 0x%p\n",
 			__FUNCTION__, fn);