Bluetooth: fix HCI ISO Data packets fragmentation

this commit partially reverts e460847b

According to Core Spec 5.4, Vol. 4, Part E, 5.4.5, Data_Total_Length field
contains length of the whole packet including optional fields of SDU
(Time_Stamp, Packet_Sequence_Number, ISO_SDU_Length, RFU and
Packet_Status_Flag). In Zephyr Host, Data_Total_Length value
is stored in bt_dev.le.iso_mtu field.
Therefore, there is no need to calculate iso_hdr_len(),
this length is already taken into account in conn_mtu(conn).

This commits removes iso_hdr_len() function and fixes
calculation of HCI ISO Data packet length calculations.

Signed-off-by: Ivan Iushkov <ivan.iushkov@nordicsemi.no>