Only get configuration once in Type-C Current mode

Before, we were doing the O(1) configuration loading operation every
15 ms, which just made me feel dirty.  Now we do it only the first time
we check the available Type-C Current, and then cache its location.
This works because it never moves without rebooting into setup mode.

src/device_policy_manager.c

@@ -116,8 +116,15 @@ void pdb_dpm_get_sink_capability(union pd_msg *cap)
 
 bool pdb_dpm_evaluate_typec_current(void)
 {
-    /* Get the current configuration */
-    struct pdb_config *cfg = pdb_config_flash_read();
+    static bool cfg_set = false;
+    static struct pdb_config *cfg = NULL;
+
+    /* Only get the configuration the first time this function runs, since its
+     * location will never change without rebooting into setup mode. */
+    if (!cfg_set) {
+        cfg = pdb_config_flash_read();
+        cfg_set = true;
+    }
 
     /* If we have no configuration or don't want 5 V, Type-C Current can't
      * possibly satisfy our needs */