ahci led: reset spins.

[ori@eigenstate.org]

On one of my machines, there was aa fairly constant load from the
ialed kproc, because it was stuck in a loop resetting the ahci
controller. The controller never came back.

I'm guessing the controller was confused because there are no
drives attached. I added a timeout as a workaround.

The AHCI spec says:

	If the HBA has not cleared GHC.HR to ‘0’ within 1 second of
	software setting GHC.HR to ‘1’, the HBA is in
	a hung or locked state.

And I've tested on a couple of machines, both with and without
drives attached, and it seems to work --  but given that people
seem to think accessing disks is important, more testing should
probably be done before committing:

diff -r c4896008f196 sys/src/9/pc/sdiahci.c
--- a/sys/src/9/pc/sdiahci.c	Fri Nov 15 13:26:25 2019 -0800
+++ b/sys/src/9/pc/sdiahci.c	Sat Nov 16 20:23:26 2019 -0800
@@ -1215,13 +1215,20 @@
 ahciencreset(Ctlr *c)
 {
 	Ahba *h;
+	int ndelay;
 
 	if(c->enctype == Eesb)
 		return 0;
 	h = c->hba;
 	h->emctl |= Emrst;
-	while(h->emctl & Emrst)
+	ndelay = 0;
+	while(h->emctl & Emrst){
+		if(ndelay++ > 1000){
+			print("hung ctlr: %s\n", Tname(c));
+			return -1;
+		}
 		delay(1);
+	}
 	return 0;
 }
 
@@ -1403,7 +1410,8 @@
 	memset(map, 0, sizeof map);
 	for(i = 0; i < niactlr; i++)
 		if(iactlr[i].enctype != 0){
-			ahciencreset(iactlr + i);
+			if(ahciencreset(iactlr + i) == -1)
+				continue;
 			map[i] = 1;
 			j++;
 		}