usb: gadget: f_cdev: Fix memory leak in creating debugfs dir

Driver creates two instances dun and nmea and saves debugfs
dir handle into a global variable i.e. debugfs.debugfs_root.
As a result debugfs_root gets overwritten when second instance
gets created. Fix this issue by saving debugfs dir handle to
cdev instance.

Change-Id: Id4295f021582a0ba677e37755b8bf27656fbcb6f
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>