USB: gadget: f_mass_storage::fsg_bind(): fix error handling

Contrary to the comment in fsg_add, fsg_bind calls fsg_unbind on errors,
which decreases refcount and frees the fsg_dev structure, causing trouble
when fsg_add does the same.

Fix it by simply leaving up cleanup to fsg_add().

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Acked-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>