Avoid stringifying condition too early

It's better if the macro receives the condition as an expression rather than a
string - that way it can choose to use it as is or stringify it. Also, the
documentation states that the parameter is an expression, not a string.
diff --git a/include/mbedtls/platform_util.h b/include/mbedtls/platform_util.h
index 1ac16d4..105b43c 100644
--- a/include/mbedtls/platform_util.h
+++ b/include/mbedtls/platform_util.h
@@ -87,7 +87,7 @@
     do {                                            \
         if( !(cond) )                               \
         {                                           \
-            MBEDTLS_PARAM_FAILED( #cond );          \
+            MBEDTLS_PARAM_FAILED( cond );           \
             return( ret );                          \
         }                                           \
     } while( 0 )
@@ -97,7 +97,7 @@
     do {                                            \
         if( !(cond) )                               \
         {                                           \
-            MBEDTLS_PARAM_FAILED( #cond );          \
+            MBEDTLS_PARAM_FAILED( cond );           \
             return;                                 \
         }                                           \
     } while( 0 )