hostapd: fix segfault in wpa_supplicant ubus

hostapd: fix segfault in wpa_supplicant ubus

When introducing ubus reload support, ubus initialization was moved
to the service level instead of being carried out when adding a BSS
configuration. While this works when using wpa_supplicant in that way,
it breaks the ability to run wpa_supplicant on the command line, eg.
for debugging purposes.
Fix that by re-introducing ubus context intialization when adding
configuration.

Reported-by: @PolynomialDivision https://github.com/openwrt/openwrt/pull/2417
Fixes: 60fb4c92b6 ("hostapd: add ubus reload")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>