mac80211: b43: improve PCIe host controller workaround

This is only needed by BCM4716 and not by BCM4706, I haven't seen any
problems there. Adds the write flush for some more parts where the phy
control regs are written.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38977 3c298f89-4303-0410-b956-a3cf2f4a3e73