mesh: Add missing action to cancel timer

mesh: Add missing action to cancel timer

IEEE Std 802.11-2012 Table 13-2, MPM finite state machine requires to
clear retryTimer when CNF_ACPT event occurs in OPN_SNT state which is
missing, so add it to comply with the standard.

This was found while debugging an MTK issue and this commit fixes a
potential issue that mesh sends invalid event (PLINK_OPEN) which will
lead another invalid timer register such as MeshConfirm Timer. This
behaviour might lead to undefined mesh state.

Signed-off-by: Peter Oh <poh@qca.qualcomm.com>