crypto: qat - reduce size of mapped region

Restrict size of field to what is required by the operation.

This issue was detected by smatch:

    drivers/crypto/qat/qat_common/qat_asym_algs.c:328 qat_dh_compute_value() error: dma_map_single_attrs() '&qat_req->in.dh.in.b' too small (8 vs 64)

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>