Sanitize ring_num value coming from configuration (and potentially
from user space) before it is used as index in the banks array.

This issue was detected by smatch:

    drivers/crypto/qat/qat_common/adf_transport.c:233 adf_create_ring() warn: potential spectre issue 'bank->rings' [r] (local cap)
Signed-off-by: Adam Guerin <adam.guerin@intel.com>
Reviewed-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Signed-off-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>