tools: usb: testusb: change the help text

The 'length' is the transfer length, not the packet size, so
change the help text.
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Cc: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

tools/usb/testusb.c

@@ -455,7 +455,7 @@ int main (int argc, char **argv)
 			"\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-s transfer length	default 512\n"
 			"\t-g sglen		default 32\n"
 			"\t-v vary			default 512\n",
 			argv[0]);