crypto: aesni - Merge GCM_ENC_DEC

The GCM_ENC_DEC routines for AVX and AVX2 are identical, except they
call separate sub-macros.  Pass the macros as arguments, and merge them.
This facilitates additional refactoring, by requiring changes in only
one place.

The GCM_ENC_DEC macro was moved above the CONFIG_AS_AVX* ifdefs,
since it will be used by both AVX and AVX2.
Signed-off-by: NDave Watson <davejwatson@fb.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>