- loaddevdesc() should request 18 bytes as per the spec.  this
avoids the problem of getting just a 17 byte descriptor.  

- loaddevstr() should request 256 bytes, as per standard.
respect the length embedded in the string.

- loaddevstr() should set a language id.  this is important for
some devices.

Reference: /n/atom/patch/applied/usbshortdesc2
Date: Wed Mar  5 14:59:29 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/usb/lib/dev.c	Wed Mar  5 14:59:22 2014
+++ /sys/src/cmd/usb/lib/dev.c	Wed Mar  5 14:59:23 2014
@@ -181,16 +181,16 @@
 }
 
 static char*
-mkstr(uchar *b)
+mkstr(uchar *b, int n)
 {
 	Rune r;
 	char *us;
 	char *s;
 	char *e;
-	int n;
 
-	n = b[0]+2;
-	if((n & 1) != 0)
+	if(n > b[0])
+		n = b[0];
+	if(n <= 2 || (n & 1) != 0)
 		return strdup("none");
 	n = (n - 2)/2;
 	b += 2;
@@ -218,10 +218,8 @@
 	else
 		langid = buf[3]<<8 | buf[2];
 	nr = usbcmd(d, type, Rgetdesc, Dstr<<8|sid, langid, buf, sizeof(buf));
-	if(nr < 2)
-		return estrdup("none");
 
-	return mkstr(buf);
+	return mkstr(buf, nr);
 }
 
 int
@@ -233,7 +231,6 @@
 	Ep *ep0;
 
 	type = Rd2h|Rstd|Rdev;
-	nr = sizeof(buf);
 	memset(buf, 0, Ddevlen);
 	if((nr = usbcmd(d, type, Rgetdesc, Ddev<<8|0, 0, buf, Ddevlen)) < 0)
 		return -1;