This add 5GHz support for N-PHY to b43, thanks Rafał Miłecki.

b43_wflush16 was put into an extra function because it is pretty big in asm.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

backport: r41900


git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42021 3c298f89-4303-0410-b956-a3cf2f4a3e73