nusb/ether: fix wrong size check causing odd sized packets to be discarded (thanks mischief!)

ethernet packets with sizes that where not multiples of 4 where
discarded because the check uses the smsc frame size instead of
the payload size. when a usb read returns just one packet, theres
no next frame header and the calculated frame size is bigger than
the usb read which caused the whole packet to be discarded as invalid.

thanks to mischief for testing and debugging!

sys/src/cmd/nusb/ether/smsc.c

@@ -219,7 +219,7 @@ smscread(Dev *ep, uchar *p, int plen)
 	hd = GET4(bin);
 	n = hd >> 16;
 	m = (n + 4 + 3) & ~3;
-	if(n < 6 || m > nbin){
+	if(n < 6 || n > nbin-4){
 		nbin = 0;
 		return 0;
 	}