hostapd: fix handling of the channel utilization options

hostapd: fix handling of the channel utilization options

Commit 0a7657c ("hostapd: add channel utilization as config option") added the
two new uci options bss_load_update_period and chan_util_avg_period. However,
the corresponding "config_add_int" calls for these options weren't added, so
attempting to actually use these options and change their values is bound to
fail - they always stay at their defaults. Add the missing code to actually
make these options work.

Fixes: 0a7657c ("hostapd: add channel utilization as config option")
Signed-off-by: Timo Sigurdsson <public_timo.s@silentcreek.de>
(cherry picked from commit 85ce590705072be78c3ef7dc6b64e3b1facc892b)