wifi: mac80211: use cfg80211_chandef_primary_freq()

Instead of calculating the new primary 40/80/160 MHz
center frequency here, use the new helper function from
cfg80211.

Link: https://msgid.link/20240129194108.eb59d6433d18.I74b745f0d1a32e779fb25d50c56407be7c35b840@changeid


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