wl1251: fix null data for IBSS

wl1251: fix null data for IBSS

Fix the WARN below by not calling ieee80211_nullfunc_get() in IBSS mode,
but setting up empty template the same way wl12xx driver does.

WARNING: CPU: 0 PID: 914 at net/mac80211/tx.c:2750 ieee80211_nullfunc_get+0xc0/0xd0 [mac80211]()
Modules linked in: wl1251_sdio wl1251 mac80211 cfg80211
...
[<c00439c0>] (warn_slowpath_null)
[<bf0bdfdc>] (ieee80211_nullfunc_get [mac80211])
[<bf134774>] (wl1251_op_bss_info_changed [wl1251])
[<bf099e14>] (ieee80211_bss_info_change_notify [mac80211])
...

Also perform join command regardless of bss_type as that seems to be
required for proper operation.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Tested-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>