net80211: fix checks for (*iv_preamble_update)/(*iv_ht_protmode_update)

Both vap_update_preamble() and vap_update_ht_protmode() also check for
(*iv_erp_protmode_update)() rather than (*iv_preamble_update)()
or (*iv_ht_protmode_update)() before calling the later.
Use the appropriate NULL-function-pointer checks before calling it.

All seem unused currently so no functional changes expected.

Fixes: f1481c8d3b58e
Reviewed by: cc
Differential Revision: https://reviews.freebsd.org/D43655

(cherry picked from commit 48d689d6cabe41f9c04e75b774ef5b3e45b94469)