Don't try to unstall USB media when it's unnecessary.

Don't try to unstall USB media when it's unnecessary.

[ori]

I've got a USB SD card reader that blocks boot for minutes. It seems like
we're trying to unstall it repeatedly as part of the boot process, and each
unstall can take tens of seconds. We don't need to unstall it, because the
device isn't stalled: It's happily reporting that there's no SD card plugged
in.

This patch adds a check to skip unstalling if we're in that case. There are
probably a few more errors that we could skip trying to unstall on.

This brings boot times from potentially multiple minutes to a few seconds.

Let me know if this looks sane to commit.

--- a/sys/src/cmd/nusb/disk/disk.c	Sun Dec 23 22:43:29 2018 +0100
+++ b/sys/src/cmd/nusb/disk/disk.c	Sun Dec 23 19:12:05 2018 -0800
@@ -361,6 +361,15 @@
 	return 0;
 }
 
+static int
+needunstall(int n)
+{
+	char buf[128];
+
+	rerrstr(buf, sizeof(buf));
+	return n < 0 || strstr(buf, "medium not present") == nil;
+}
+
 
 /*
  * called by SR*() commands provided by scuzz's scsireq
@@ -412,15 +421,14 @@
 			if (n >= 0 && n < nio)	/* didn't fill data->p? */
 				memset(data->p + n, 0, nio - n);
 		}
-		nio = n;
 		if(diskdebug)
 			if(n < 0)
 				fprint(2, "disk: data: %r\n");
 			else
-				fprint(2, "disk: data: %d bytes\n", n);
-		if(n <= 0)
-			if(data->write == 0)
-				unstall(dev, ums->epin, Ein);
+				fprint(2, "disk: data: %d bytes (nio: %d)\n", n, nio);
+		nio = n;
+		if(n <= 0 && (needunstall(n) || data->write == 0))
+			unstall(dev, ums->epin, Ein);
 	}
 
 	/* read the transfer's status */