Bluetooth: controller: Fix Tx Buffer Overflow

Fix Tx Buffer Overflow caused by uninitialized node_tx
memory being used by ULL ISR context due to Compiler
Instructions Reordering in the use of MFIFO_ENQUEUE.

The MFIFO last index was committed before the data element
was stored in the MFIFO due to Compiler Instructions
Reordering.

This is fixed now by adding a Data Memory Barrier
instruction alongwith a compiler memory clobber.

Fixes #30378.

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