net: usb: rmnet: Fix bug in handling removal of debugfs files

RmNet supports multiple driver instances. Every driver instance creates
a debugfs entry. Removal of same debugfs entry by every driver instance
during device disconnect causes crash. Fix the crash by making sure that
driver only removes the debugfs entry which was created by it.

Change-Id: I6bcd07f07f3faecffe7f4b908d751c769d830ad1
CRs-Fixed: 356385
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>