Bluetooth: SMP: Fix min key size for LE Secure Connections Only mode

Secure Connection Only mode requires use of LE Security mode 1 level 4
which mandates 128 encryption key size.

Defaults in Kconfig are set from top-to-bottom and this resulted in
7 bytes key being forced. What is worse, user cannot override this
from prj.conf file since BT_SMP_MIN_ENC_KEY_SIZE is hidden config
if BT_SMP_SC_ONLY is enabled.

Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>