Fix sign warning on windows build for bitforce.
diff --git a/driver-bitforce.c b/driver-bitforce.c
index 606ec2e..ae9fa01 100644
--- a/driver-bitforce.c
+++ b/driver-bitforce.c
@@ -154,7 +154,7 @@ static int bitforce_autodetect_ftdi(void)
char **bufptrs;
char *buf;
int found = 0;
- int i;
+ DWORD i;
FT_STATUS ftStatus;
DWORD numDevs;