Bluetooth: Controller: nRF51x: Fix regression in encrypted connection

Fix regression in encrypted connection introduced in
commit f3deccda ("Bluetooth: Controller: CCM read data
to early when DF enabled on PHY 1M").

Due to this nRF51x SoC hang waiting to encrypt and/or
check MIC.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>
(cherry picked from commit b9a7a645)