UCP/CORE: Fix async lock assertion failure upon receiving speed change speed_change_event

IBV_EVENT_DEVICE_SPEED_CHANGE event is popped from the UCT worker progress
queue. ucp_worker_progress() does not acquire the worker async lock in
SINGLE mode, while ucp_worker_iface_handle_port_speed_event() checks
the lock to be held, which causes an assertion failure.

This commit explicitly blocks the worker async context.

Signed-off-by: Zhenlong Ma <mazhenlong321@gmail.com>