Bluetooth: Controller: Fix re-encryption procedure

During implementation of alternate encryption procedure
usable in nRF51, commited in
c41d3edd, re-encryption
procedure was broken. This commit fixes the issue, and
now re-encryption should work on both nRF51 and nRF52.

Change-id: Ia41200f42b1d46e1f3f35ff44b582d4ffcc5f4fa
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>