When the driver request a API gets RT_NULL which return value is ptr,
they could not know why get a RT_NULL.
some API return RT_NULL, is not error internal maybe, it just not
supported for this platform, but the driver still could work ok,
the API can return (RT_NULL + -RT_EEMPTY) to driver.
on the other hand, the driver can do more behaviors by error no.
When the API return the -RT_EBUSY, driver can wait for a moment and retry.
When the API return the -RT_ENOSYS, driver can try the next mode or request's name.
Signed-off-by: GuEe-GUI <wusongjie@rt-thread.com>
f9b632d52d