ath5k: Fix beaconing in mesh mode

This patch fixes the oops below when attempting to bring up a mesh
interface on ath5k hardware.

[  128.933099] kernel BUG at drivers/net/wireless/ath/ath5k/base.c:197!
[  128.933099] invalid opcode: 0000 [#1]
(...)
[  128.933099] Call Trace:
[  128.933099]  [<c83b77fa>] ? ath5k_beacon_update+0x57/0x1f8 [ath5k]
[  128.933099]  [<c02d9a40>] ? __sysfs_add_one+0x28/0x76
[  128.933099]  [<c83b830e>] ? ath5k_bss_info_changed+0x13f/0x173
[ath5k]
[  128.933099]  [<c82ff629>] ? ieee80211_config_beacon+0xc0/0x17e
[mac80211]
[  128.933099]  [<c82f073e>] ?
ieee80211_bss_info_change_notify+0x182/0x18b [mac80211]
[  128.933099]  [<c83b81cf>] ? ath5k_bss_info_changed+0x0/0x173 [ath5k]
[  128.933099]  [<c82ff6d6>] ? ieee80211_config_beacon+0x16d/0x17e
[mac80211]
[  128.933099]  [<c82ff753>] ? ieee80211_add_beacon+0x34/0x39 [mac80211]
[  128.933099]  [<c830a4ed>] ? ieee80211s_init+0xf8/0x10f [mac80211]
[  128.933099]  [<c830a5df>] ? ieee80211_mesh_init_sdata+0xdb/0x154 [mac80211]

Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>