mISDN: fix possible use-after-free in HFC_cleanup()

In the Linux kernel, the following vulnerability has been resolved:

mISDN: fix possible use-after-free in HFC_cleanup()

This module's remove path calls del_timer(). However, that function

does not wait until the timer handler finishes. This means that the

timer handler may still be running after the driver's remove function

has finished, which would result in a use-after-free.

Fix by calling del_timer_sync(), which makes sure the timer handler

has finished, and unable to re-schedule itself.