testusb: expose looping forever option "l" to user

The testusb.c tool has support for looping forever implemented, which
may be useful for stress test, yet it is not exposed to the user, so
even though the code is there, it cannot be used.  This commit adds
"l" to the set of options handled by the application which enables
the feature.

Also, I collate help information for each command line option to make
it easier to use for novice.
Signed-off-by: NDu Changbin <changbinx.du@intel.com>
Acked-by: NMichal Nazarewicz <mina86@mina86.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

tools/usb/testusb.c

@@ -425,7 +425,7 @@ int main (int argc, char **argv)
 	/* for easy use when hotplugging */
 	device = getenv ("DEVICE");
 
-	while ((c = getopt (argc, argv, "D:aA:c:g:hns:t:v:")) != EOF)
+	while ((c = getopt (argc, argv, "D:aA:c:g:hlns:t:v:")) != EOF)
 	switch (c) {
 	case 'D':	/* device, if only one */
 		device = optarg;
@@ -468,10 +468,21 @@ int main (int argc, char **argv)
 	case 'h':
 	default:
 usage:
-		fprintf (stderr, "usage: %s [-n] [-D dev | -a | -A usbfs-dir]\n"
-			"\t[-c iterations]  [-t testnum]\n"
-			"\t[-s packetsize] [-g sglen] [-v vary]\n",
-			argv [0]);
+		fprintf (stderr,
+			"usage: %s [options]\n"
+			"Options:\n"
+			"\t-D dev		only test specific device\n"
+			"\t-A usbfs-dir\n"
+			"\t-a		test all recognized devices\n"
+			"\t-l		loop forever(for stress test)\n"
+			"\t-t testnum	only run specified case\n"
+			"\t-n		no test running, show devices to be tested\n"
+			"Case arguments:\n"
+			"\t-c iterations	default 1000\n"
+			"\t-s packetsize	default 512\n"
+			"\t-g sglen	default 32\n"
+			"\t-v vary		default 512\n",
+			argv[0]);
 		return 1;
 	}
 	if (optind != argc)