usb: gadget: f_cdev: Fix NULL pointer dereference in cser_free_inst

If f_cdev_alloc() fails it frees the port context and set_inst_name()
call back returns with error. As a result free_func_inst() call back
is called which is dereferencing port context from f_cdev_opts context
which results into NULL ptr dereference. Fix the issue by adding NULL
check for port context pointer in f_cdev_opts context.

Change-Id: I69828761be0a9f7df714eec34894c13f762dcc43
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>