Bluetooth: Controller: Fix suspicious use of sizeof

Fix Coverity, sizeof not portable, defect; by explicitly
using sizeof(void *).

suspicious_sizeof:
Passing argument mem_head of type void ** and argument 4U
/* sizeof (mem_head) */ to function memcpy is suspicious.
In this case, sizeof (void **) is equal to sizeof
(void *), but this is not a portable assumption.

Change-id: I4b4776466e16020876500feba0141985b8581017
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>