[crypto] Support in-place decryption for GCM ciphers

[crypto] Support in-place decryption for GCM ciphers

The hash calculation is currently performed incorrectly when
decrypting in place, since the ciphertext will have been overwritten
with the plaintext before being used to update the hash value.

Restructure the code to allow for in-place encryption and decryption.
Choose to optimise for the decryption case, since we are likely to
decrypt much more data than we encrypt.

Signed-off-by: Michael Brown <mcb30@ipxe.org>