Bluetooth: Rename smp_key_size to enc_key_size

This makes clear that this is the size of the key used to
encrypt the link.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@openbossa.org>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>