crypto: engine - Fix struct crypto_engine_op doc

Remove redundant underscore and fix some grammar in prepare_request doc.
Signed-off-by: David Yang <mmyangfl@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

include/crypto/engine.h

@@ -78,7 +78,7 @@ struct crypto_engine {
 
 /*
  * struct crypto_engine_op - crypto hardware engine operations
- * @prepare__request: do some prepare if need before handle the current request
+ * @prepare_request: do some preparation if needed before handling the current request
  * @unprepare_request: undo any work done by prepare_request()
  * @do_one_request: do encryption for current request
  */