nfc: hci: shdlc: Stop timers and work before freeing context

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

nfc: hci: shdlc: Stop timers and work before freeing context

llc_shdlc_deinit() purges SHDLC skb queues and frees the llc_shdlc

structure while its timers and state machine work may still be active.

Timer callbacks can schedule sm_work, and sm_work accesses SHDLC state

and the skb queues. If teardown happens in parallel with a queued/running

work item, it can lead to UAF and other shutdown races.

Stop all SHDLC timers and cancel sm_work synchronously before purging the

queues and freeing the context.

Found by Linux Verification Center (linuxtesting.org) with SVACE.