don't hide -1 in a ulong

Reference: /n/atom/patch/applied/smbussign
Date: Sat Jan 11 21:46:54 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/9/pc/piix4smbus.c	Sat Jan 11 21:46:28 2014
+++ /sys/src/9/pc/piix4smbus.c	Sat Jan 11 21:46:28 2014
@@ -171,6 +171,7 @@
 SMBus*
 piix4smbus(void)
 {
+	int b;
 	Pcidev *p;
 	static SMBus *s;
 
@@ -192,12 +193,13 @@
 	s->base = pcicfgr32(p, SMBbase) & ~1;
 print("SMB base from bios is 0x%lux\n", s->base);
 	if(ioalloc(s->base, 0xd, 0, "piix4smbus") < 0){
-		s->base = ioalloc(-1, 0xd, 2, "piix4smbus");
-		if(s->base < 0){
+		b = ioalloc(-1, 0xd, 2, "piix4smbus");
+		if(b < 0){
 			free(s);
 			print("piix4smbus: can't allocate io port\n");
 			return nil;
 		}
+		s->base = b;
 print("SMB base ialloc is 0x%lux\n", s->base);
 		pcicfgw32(p, SMBbase, s->base|1);
 	}