Bluetooth: Controller: Rename to BT_CTLR_ISO_TX_PDU_BUFFERS

Rename the internal BT_CTLR_ISO_TX_BUFFERS to
BT_CTLR_ISO_TX_PDU_BUFFERS correct represent the number
of actual ISO PDU buffers allocated in the Controller.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>