USB: gadget: serial: Fix debugfs crash

Serial function driver creates debugfs files even though ports
are not allocated. Fetching/reading those files without allocation
of ports leads to crash. Check port allocation before creating
the files.

CRs-Fixed: 1003498
Change-Id: I85b050a261cca6f961d5d9058efb8b7facf242ce
Signed-off-by: Arumuga Durai A <cadurai@codeaurora.org>