sr: split driver init into init() and scan()

sr: split driver init into init() and scan()

init() now only does whatever administrative stuff it needs (typically not
much), and returns an error code.

scan() can be called multiple times during the life of an application, and
returns a GSList of struct sr_dev_inst * of devices found during that scan.
The instances are a copy of the ones stored in the driver's own instance
list, to be freed by the caller with g_slist_free() only.

The scan() call can be passed a GSList of struct sr_hwopt *, to direct the
scanning.