Bluetooth: SMP: Fix parallel pairing needing DHKey at the same time

Fix parallel pairing procedures using LE SC requiring the DHKey
calculation at the same time. This would otherwise end all other
pairing procedures with the SMP error code "unspecified" since
the call to bt_gen_dh_key would fail.

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