ath9k_htc: Add support for mesh interfaces

More specifically, enable AP-style beaconing on mesh
ifaces and change the hw capabilities to reflect mesh
support.

Coexistence with a virtual STA interface was tested as
working fine.

Signed-off-by: Javier Cardona <javier@cozybit.com>
[rebase, add iface combinations]
Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>