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>

TODO: test interface combinations:
peer, associate, and ping.
	0. mesh
	1. mesh + mesh
	2. mesh + sta
	3. mesh + ap

edit if_limits[] in
drivers/net/wireless/ath/ath9k/htc_drv_init.c with what
does / doesn't work.

Also, do we even need num_mbss_vifs any more? This should
be expressed in the if_limits[]?
ath9k_htc: Claim support for mgmt frame protection

Advertise support for management frame protection in hardware.

Signed-off-by: Javier Cardona <javier@cozybit.com>