bluetooth: host: smp: Add runtime check for central-specific path

Added run-time BT_CENTRAL role check for the path that was
central specific and did not have such check.

When multi-role BT device tried to pair without bonding (peripheral role)
while already previously bonded with the same device on another
Bluetooth identity, pairing failed.
It executed central-specific code, which should not be executed in case
when the device acts as peripheral (as it is even opt-out from code when
CONFIG_BT_CENTRAL is not enabled).

Signed-off-by: Mateusz Kapala <mateusz.kapala@nordicsemi.no>