Bluetooth: Classic: SMP: Avoid derived LE keys be added multiple times

In current implementation, the flag `local_dist` will be cleared when
the distributed key frame is performed if the local is the SMP
initiator. After the distributed key is sent out, the function
`smp_pairing_br_complete()` will be called if all bits of `local_dist`
are cleared.

It causes the function `smp_pairing_br_complete()` will be called
multiple times.

Add a flag `local_distributed` to flag the all sent keys. Add only the
flag `local_distributed` is not set, preform the key distribution
frame.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>