USB: gadget: save driver name before registering it

g_dnl_register() currently first attempts to register a composite
driver by name, and then saves the driver name once it's registered.
Internally to the registration code, g_dnl_do_config() is called and
attempts to compare the composite device's name with the list of known
device names. This fails since the composite device's name has not yet
been stored. This means that the first time "ums 0 0" is run, it fails,
but subsequent attempts succeed.

Re-order the name-saving and registration code to solve this.

Fixes: e5b834e07f51 ("USB: gadget: added a saner gadget downloader registration API")
Signed-off-by: Stephen Warren <swarren@nvidia.com>