usb: gadget: u_rmnet_ctrl_qti: Add NULL check for port

Add NULL check for the port in gqti_ctrl_connect() and
gqti_ctrl_disconnect().

Change-Id: I97c5f2454c9ed97d43a2588f31ea8d010a823a08
Signed-off-by: Saket Saurabh <ssaurabh@codeaurora.org>