sigrok-cli.c: Fix memory leak in parsing of hardware options

During parsing of hardware options, hash_to_hwopt() creates a GSList that is
passed to device_scan(). The list is discarded by device_scan() and never freed.
This is identified by valgrind as "definitely lost".

Call g_slist_free_full() on the list when it is no longer needed.

Signed-off-by: Alexandru Gagniuc <redacted>