Out-of-bounds access possible if ctx.qat_cipher_alg has invalid value.
This should never happen at this point on data path, but fix for safety.
Coverity issue: 143458, 143465
Fixes: d18ab45f7654 ("crypto/qat: support DOCSIS BPI mode")
Signed-off-by: Fiona Trahe <fiona.trahe@intel.com>