ath9k: add support for 802.11n bonded out AR2427

Some single chip family devices are sold in the market with
802.11n bonded out, these have no hardware capability for
802.11n but ath9k can still support them. These are called
AR2427.

Cc: stable@kernel.org
Reported-by: Rolf Leggewie <bugzilla.kernel.org@rolf.leggewie.biz>
Tested-by: Bernhard Reiter <ockham@raz.or.at>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>