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: Du Changbin <changbinx.du@intel.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>