usb: gadget: u_smd: Check if SMD channel exists before queueing pull

Currently driver queues tx_pull_work without checking if port
channel exists. If it becomes NULL in some scenario, then pull
work is queued infinitely which leads to watchdog timeout.
Fix this by ensuring that SMD channel exists before queueing
pull.

Change-Id: If02aabda357040e3009ef10e7922d92a6e48996f
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>