Bluetooth: controller: Fix rx_ftr and extra overlapping memory

The the footer structure and extra bytes (rssi, resolving index etc.)
were overlapping in memory, rx_ftr was moved into the header, but the
extra and footer are still being read from the same place, extra was
written to the end. So this avoided memory corruption, but reading extra
reads wrong information.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>
Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>