usb: Set category and description of the MTP device

It's a storage device, so let's classify it accordingly. And
while we're at it, also add a short description for people who
do not know what MTP means.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>