Ath5k: add AP mode

Add support for AP mode. This involves:
- enablement in ath5k_beacon_config -- initialize beacon timer
- add AP to the supported modes in ath5k_add_interface
- handle beacon change even for AP in ath5k_config_interface
- remove useless test for IBSS in ath5k_beacon_update

Note that it doesn't enable the AP mode for the driver. It must
be enabled by NL80211_IFTYPE_AP bit added to interface_modes.

v2:
Fixed opmode constant (IEEE80211_ to NL80211_)

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Nick Kossifidis <mickflemm@gmail.com>
Cc: Luis R. Rodriguez <mcgrof@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>