BLE: Don't set SC bit in AuthReq for Legacy Devices

Disable SC bit for legacy LE devices (< 4.2) since as per earlier
spec, this BIT should be 0 and ignored by remote device. Some remote
legacy devices which don't follow the approach of ignoring these bits
(if set) go in bad state and result in SMP pairing failure.

Test: Pairing succeeds with remote device after resettings SC bit.

Fixes: 70259025
Change-Id: I35838c60fb08916a141f764b3e1922d277b99f28