ao_pulse: fix subscribe_cb using wrong bitmask for event type

PA_SUBSCRIPTION_MASK_SINK is a pa_subscription_mask constant used when
registering with pa_context_subscribe(), not a
pa_subscription_event_type mask. Masking the event type with it would
never match PA_SUBSCRIPTION_EVENT_NEW or PA_SUBSCRIPTION_EVENT_REMOVE,
causing hotplug events to be silently dropped.

Fixes: e01750020dcc9d49e6f32d513f1b40cd30148d91