Bluetooth: Correct packet len calculation

Remove unneeded skb_pull and correct packet length calculation
removing magic number. Move BT_DBG after len check otherwise
it could possibly access wrong memory.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>