Bluetooth: hci_event: fix MTU for BN == 0 in CIS Established

BN == 0x00 in CIS Established means no isochronous data for the
corresponding direction (Core v6.1 pp. 2394). In this case SDU MTU
should be 0.

However, the specification does not say the Max_PDU_C_To_P or P_To_C are
then zero.  Intel AX210 in Framed CIS mode sets nonzero Max_PDU for
direction with zero BN.  This causes failure later when we try to LE
Setup ISO Data Path for disabled direction, which is disallowed (Core
v6.1 pp. 2750).

Fix by setting SDU MTU to 0 if BN == 0.

Fixes: 2be22f19 ("Bluetooth: hci_event: Fix parsing of CIS Established Event")
Signed-off-by: Pauli Virtanen <pav@iki.fi>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>