psa: Add initializers for cipher operation objects

Add new initializers for cipher operation objects and use them in our
tests and library code. Prefer using the macro initializers due to their
straightforwardness.
diff --git a/include/psa/crypto_struct.h b/include/psa/crypto_struct.h
index 5eb2624..ee3ecd7 100644
--- a/include/psa/crypto_struct.h
+++ b/include/psa/crypto_struct.h
@@ -145,6 +145,13 @@
     } ctx;
 };
 
+#define PSA_CIPHER_OPERATION_INIT {0, 0, 0, 0, 0, 0, {0}}
+static inline struct psa_cipher_operation_s psa_cipher_operation_init( void )
+{
+    const struct psa_cipher_operation_s v = PSA_CIPHER_OPERATION_INIT;
+    return( v );
+}
+
 #if defined(MBEDTLS_MD_C)
 typedef struct
 {