crypto: aesni - Merge encode and decode to GCM_ENC_DEC macro

Make a macro for the main encode/decode routine.  Only a small handful
of lines differ for enc and dec.   This will also become the main
scatter/gather update routine.

Signed-off-by: Dave Watson <davejwatson@fb.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>