Restore P>Q in RSA key generation (#558)

The PKCS#1 standard says nothing about the relation between P and Q
but many libraries guarantee P>Q and mbed TLS did so too in earlier
versions.

This commit restores this behaviour.
diff --git a/tests/suites/test_suite_rsa.function b/tests/suites/test_suite_rsa.function
index 8837e3a..d48bc85 100644
--- a/tests/suites/test_suite_rsa.function
+++ b/tests/suites/test_suite_rsa.function
@@ -678,6 +678,7 @@
     if( result == 0 )
     {
         TEST_ASSERT( mbedtls_rsa_check_privkey( &ctx ) == 0 );
+        TEST_ASSERT( mbedtls_mpi_cmp_mpi( &ctx.P, &ctx.Q ) > 0 );
     }
 
 exit: