rtl8xxxu: Introduce set_tx_power() fileop and a new 8723b dummy derivative

The 8723b series is significantly different from the older generation
in this sense. So far the 8723b version doesn't do anything useful.

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