Bluetooth: Audio: Fix issue with supporting bidir CIS in ASCS

If 2 ASEs shared the same CIS and the first ASE had the CIS
connected before the second ASE was QoS configured, then
the CIS was missing either the TX or RX pointer, causing
it to be considered unidirectional by the ISO layer.

This commit fixes this issue by configuring the (static)
pointers at an earlier time, so that the RX and TX QoS
pointers are always valid.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>