used signed comparison for ticks

Reference: /n/atom/patch/applied2013/sdscsifisstick
Date: Sat Sep 21 21:06:27 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/9/port/sdscsifis.c	Sat Sep 21 21:06:27 2013
+++ /sys/src/9/port/sdscsifis.c	Sat Sep 21 21:06:27 2013
@@ -196,7 +196,7 @@
 {
 	int d;
 
-	d = TK2MS(tk - Ticks);
+	d = TK2MS((long)(tk - Ticks));
 	if(d <= 0)
 		return;
 	if(d < ms)