rtl8xxxu: Add rtl8723bu_phy_init_antenna_selection()

So far this is just for 8723BU. It includes writing to a number of
registers I have seen no description for so far.

0x0064
0x0930
0x0944

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