Bluetooth: Simplify logic when checking SMP_FLAG_TK_VALID

This is a trivial coding style simplification by instead of having an
extra early return to instead revert the if condition and do the single
needed queue_work() call there.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>