mac80211: fix warning in 6 GHz IE addition in mesh mode

mac80211: fix warning in 6 GHz IE addition in mesh mode

The commit ac3e69b52a30 ("mac80211: add HE 6 GHz Band Capability
element") failed to check device capability before adding HE 6 GHz
capability element. Below warning is reported in 11ac device in mesh.
Fix that by checking device capability at HE 6 GHz cap IE addition
in mesh beacon and association request.

WARNING: CPU: 1 PID: 1897 at net/mac80211/util.c:2878
ieee80211_ie_build_he_6ghz_cap+0x149/0x150 [mac80211]
[ 3138.720358] Call Trace:
[ 3138.720361]  ieee80211_mesh_build_beacon+0x462/0x530 [mac80211]
[ 3138.720363]  ieee80211_start_mesh+0xa8/0xf0 [mac80211]
[ 3138.720365]  __cfg80211_join_mesh+0x122/0x3e0 [cfg80211]
[ 3138.720368]  nl80211_join_mesh+0x3d3/0x510 [cfg80211]

Fixes: ac3e69b52a30 ("mac80211: add HE 6 GHz Band Capability element")
Reported-by: Markus Theil <markus.theil@tu-ilmenau.de>
Signed-off-by: Rajkumar Manoharan <rmanohar@codeaurora.org>
Link: https://lore.kernel.org/r/1593656424-18240-1-git-send-email-rmanohar@codeaurora.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>