hostapd: Register wpa_msg callback even if only global ctrl_iface is used

hostapd: Register wpa_msg callback even if only global ctrl_iface is used

Previously, wpa_msg_register_cb() was called only from successful
completion of per-interface control interface initialization. This would
leave the callback unregistered in case only the global control
interface is used which would result in not delivering control interface
events on the global interface. Fix this by registering the callback
handler also from successful initialization of the global control
interface.

Signed-off-by: Veerendranath Jakkam <vjakkam@codeaurora.org>