usb: u_glink: Fix null pointer dereference of port

There is a possible erratic scenario where the
set_alt is not called but the disable is called.
This might lead to an invalid null access of
the port.
Fix this by checking the validity of the port
before accessing it.

Change-Id: Iab8f5570a0e9049cb5204793025a9f1c2ac3113c
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>