Bluetooth: SMP: Handle both devices initiating security

Handle case where:
 - Peripheral sends security request after master has sent pairing
   request or started encryption procedure.
   This packet can be ignored, as long as the slave has not already
   responded with pairing response.
 - Central wants to start security after peripheral initiated security
   request, return error code busy in this case

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>