Bluetooth: SMP: Refactor pairing request handling

This is in preparation for adding support to confirm incoming pairing
requests. smp_pairing_req is now more similar to smp_pairing_rsp making
code easier to follow.

Change-Id: Ie5a77ae5092b7bbf76a482d0bd49e022c4d19d70
Signed-off-by: Szymon Janc <ext.szymon.janc@tieto.com>