Bluetooth: Refactor raw socket filter into more readable code

The handling of the raw socket filter is rather obscure code and it gets
in the way of future extensions. Instead of inline filtering in the raw
socket packet routine, refactor it into its own function.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>