USB-CDC: change simple_strtol to simple_strtoul

The patch is based on commit bb9496c6f7e853e5d4edd5397c9d45f1968d623c
(done by Julia Lawall <julia@diku.dk>) from linux-2.6.git.

Since num is unsigned, it would seem better to use simple_strtoul that
simple_strtol.

Signed-off-by: Vitaly Kuzmichev <vkuzmichev@mvista.com>