wifi: mac80211: don't use sband->band early

Some drivers may (notably mt76 does) not set up sband->band
before registering, and cfg80211 will fill it in later. But
since the HT/HE capability check mac80211 required it to be
initialized already, otherwise failing things.  This really
isn't necessary though since the code is iterating the list
of bands, and has the 'band' variable available.  Fix it to
not require the sband->band to be initialized already.

Fixes: f04d2c24 ("wifi: mac80211: disallow drivers with HT wider than HE")
Reported-by: Bert Karwatzki <spasswolf@web.de>
Debugged-by: Bert Karwatzki <spasswolf@web.de>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218466
Link: https://msgid.link/20240207115920.43cbedffb5c3.I4968e12275a3f95926e3f3ccae81e50f23fe4d4d@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>