staging: ccree: simplify AEAD using local var

Make the code more readable by using a local variable
for commonly use expression in the AEAD part of the driver.

Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>