[netdrvr/usb] hso_create_bulk_serial_device(): fix a double free

hso_serial_common_free() mustn't be called if
hso_serial_common_create() fails.

Reported-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>