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

[bz]

Summary

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.

MFC after: 3 days

Test Plan

all three seem unused in the tree so no actual problem (yet)