drivers: crypto: it8xxx2_evb: fix a shadow variable error

Fix a build error:

crypto_it8xxx2_sha.c:99:26: warning: declaration of 'i' shadows a
previous local [-Wshadow]
   99 |                 for (int i = 0; i < ARRAY_SIZE(sha256_k); i++) {
      |                          ^ crypto_it8xxx2_sha.c:88:13: note:
shadowed declaration is here
   88 |         int i;

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>