rtl8xxxu: Implement 8192e specific power down sequence

This powers down the 8192e correctly, or at least to the point where
the firmware will load again, when reloading the driver module.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>