rtl8xxxu: Split rtl8xxxu_init_phy_bb() into device specific functions

This reduces the if () clutter. Longer term it probably makes sense to
split this between gen1 (8723au/8188cu/8192cu) and gen2
(8192eu/8723bu) devices.

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