Bluetooth: controller: Check minimum sizes of adv PDUs

While the maximum sizes were already correctly checked by the code, the
minimum sizes of the PDUs were not. This meant that PDUs smaller than
the minimum required length (typically 6 bytes for AdvA) were
incorrectly forwarded up to the Host.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
(cherry picked from commit 3f0d7012)