pkcs7: Use better error codes

Remove an unnecessary debug print (whoops).
Use new error code for when the x509 is expired.
When there are no signers return invalid certificate.

Signed-off-by: Nick Child <nick.child@ibm.com>

Co-authored-by: Dave Rodgman <dave.rodgman@arm.com>
Signed-off-by: Nick Child <nick.child@ibm.com>
diff --git a/library/pkcs7.c b/library/pkcs7.c
index 7976a0b..ca0170a 100644
--- a/library/pkcs7.c
+++ b/library/pkcs7.c
@@ -630,15 +630,14 @@
 
     if( pkcs7->signed_data.no_of_signers == 0 )
     {
-        ret = MBEDTLS_ERR_PKCS7_VERIFY_FAIL;
+        ret = MBEDTLS_ERR_PKCS7_INVALID_CERT;
         goto out;
     }
 
     if( mbedtls_x509_time_is_past( &cert->valid_to ) ||
         mbedtls_x509_time_is_future( &cert->valid_from ))
     {
-        printf("EXPRED\n");
-        ret = MBEDTLS_ERR_PKCS7_VERIFY_FAIL;
+        ret = MBEDTLS_ERR_PKCS7_CERT_DATE_INVALID;
         goto out;
     }