Bluetooth: avoid u128_xor() on potentially misaligned inputs

u128_xor() takes pointers to quantities that are assumed to be at least
64-bit aligned, which is not guaranteed to be the case in the smp_c1()
routine. So switch to crypto_xor() instead.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>