Bluetooth: ISO: exclude RFU bits from ISO_SDU_Length

slen contains ISO_SDU_Length (12 bits), RFU (2 bits),
Packet_Status_Flags (2 bits).

Exclude the RFU bits from hci_iso_data_len. Also add masks to the pack
macro.

Fixes: 4de0fc599eb9 ("Bluetooth: Add definitions for CIS connections")
Signed-off-by: Pauli Virtanen <pav@iki.fi>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>