crypto: aesni - make driver-gcm-aes-aesni helper a proper aead alg

Changed the __driver-gcm-aes-aesni to be a proper aead algorithm.
This required a valid setkey and setauthsize functions to be added and also
some changes to make sure that math context is not corrupted when the alg is
used directly.
Note that the __driver-gcm-aes-aesni should not be used directly by modules
that can use it in interrupt context as we don't have a good fallback mechanism
in this case.

Signed-off-by: Adrian Hoban <adrian.hoban@intel.com>
Signed-off-by: Tadeusz Struk <tadeusz.struk@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>