mac80211: backport some brcmfmac patches

There are two important patches in this patchset: updating read pointer
quicker & rework of .get_station().
There are few more upstream patches that are p2p-related and weren't
backported in this commit.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

Backport of r46084

git-svn-id: svn://svn.openwrt.org/openwrt/branches/chaos_calmer@46100 3c298f89-4303-0410-b956-a3cf2f4a3e73