wifi: mac80211: simplify HE/EHT element length functions

We don't need to pass the iftype there, we already have it
in the sdata. Simplify this code.

Link: https://msgid.link/20240129202041.5890eb1d4184.Ibce7e5abcc7887630da03ac2263d8004ec541418@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>