efifb: Fix call to wrong unregister function

platform_device_unregister() needs to unregister the device, not the
driver.

Signed-off-by: Wanlong Gao <wanlong.gao@gmail.com>
Signed-off-by: Maarten Lankhorst <m.b.lankhorst@gmail.com>
Acked-by: Andy Lutomirski <luto@mit.edu>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>