Bluetooth: controller: Use DMB instead of DSB

Use of Data Memory Barrier instruction with memory clobber
in ARM Cortex M architectures is sufficient in the
controller implementation to keep compiler data access
instructions in order so that an ISR vectoring has memory
accesses in the correct order as intented by design.

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