Bluetooth: controller: Fix uninitialized ULL reference count

Fix missing initialization of reference count in
ull_hdr_init function.

This has not caused issues so far, but when the ref member
of the struct ull_hdr if placed in the beginning of a
context that is allocated using mem_acquire function then
first few bytes used would make the ref member to have
uninitialized value when such context is allocated by
mem_acquire. First few bytes are the next pointer and free
count stored by the mem module.

The issue was discovered in subsequent commits that
restructure the ULL context structures.

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