crypto: sha256-mb - fix ctx pointer and digest copy

1. fix ctx pointer
Use req_ctx which is the ctx for the next job that have
been completed in the lanes instead of the first
completed job rctx, whose completion could have been
called and released.
2. fix digest copy
Use XMM register to copy another 16 bytes sha256 digest
instead of a regular register.

Signed-off-by: Xiaodong Liu <xiaodong.liu@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>