crypto: poly1305 - use unaligned access macros to output digest

Currently the only part of poly1305-generic which is assuming special
alignment is the part where the final digest is written.  Switch this
over to the unaligned access macros so that we'll be able to remove the
cra_alignmask.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>