Fix return values of amp_load_backend() and rot_load_backend()

The value -EINVAL is in the domain of errno.

src/amp_reg.c

@@ -388,7 +388,7 @@ int HAMLIB_API amp_load_backend(const char *be_name)
             if (be_init == NULL)
             {
                 printf("Null\n");
-                return -EINVAL;
+                return -RIG_EINVAL;
             }
 
             status = (*be_init)(NULL);
@@ -396,7 +396,7 @@ int HAMLIB_API amp_load_backend(const char *be_name)
         }
     }
 
-    return -EINVAL;
+    return -RIG_EINVAL;
 
 }
 //! @endcond

src/rot_reg.c

@@ -445,7 +445,7 @@ int HAMLIB_API rot_load_backend(const char *be_name)
             if (be_init == NULL)
             {
                 printf("Null\n");
-                return -EINVAL;
+                return -RIG_EINVAL;
             }
 
             status = (*be_init)(NULL);
@@ -453,7 +453,7 @@ int HAMLIB_API rot_load_backend(const char *be_name)
         }
     }
 
-    return -EINVAL;
+    return -RIG_EINVAL;
 
 }
 //! @endcond