crypto: qat - Fix incorrect uses of memzero_explicit

memzero_explicit should only be used on stack variables that get
zapped just before they go out of scope.

This patch replaces all unnecessary uses of memzero_explicit with
memset, removes two memzero_explicit calls altogether as the tfm
context comes pre-zeroed, and adds a missing memzero_explicit of
the stack variable buff in qat_alg_do_precomputes.  The memzeros
on ipad/opad + digest_size/auth_keylen are also removed as the
entire auth_state is already zeroed on entry.

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