Bluetooth: Fix failure to release lock in read_index_list()

If alloc_skb() fails in read_index_list() we'll return -ENOMEM without
releasing 'hci_dev_list_lock'.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>