staging: ccree: clean up struct ssi_aead_ctx

struct ssi_aead_ctx has some nested structure defined, resulting
in code accessing them to be very unreadable.

Move out the nested structure definitions out of the struct
and use the change to make the code accessing it more readable
and better coding style compliant by shortening lines and
properly matching alignment,

Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>