rtl8xxxu: Implment rtl8192e_set_tx_power()

8192eu is a 2T part, so setting TX power for path A only, as done by
rtl8723bu_set_tx_power() is not sufficient.

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