[settings] reset the platform settings file descriptor once closed (#10512)

On platform settings deinit, the file descriptor is closed but the
variable is not reset, which can lead to calling close on a stale fd
leading to bad file descriptor error in case the API is invoked
multiple times.

src/posix/platform/settings.cpp

@@ -232,6 +232,7 @@ void otPlatSettingsDeinit(otInstance *aInstance)
 
     VerifyOrExit(sSettingsFd != -1);
     VerifyOrDie(close(sSettingsFd) == 0, OT_EXIT_ERROR_ERRNO);
+    sSettingsFd = -1;
 
 exit:
     return;