bcmdhd: wait for cfg cleanup before unregistering

bcmdhd: wait for cfg cleanup before unregistering

If the p2p device is unregistered before the cfg
cleanup work item is scheduled then, the wdev
struct is corrupted and can cause the kernel panic.
wait for the work item to finish before freeing

Bug 1533639

Change-Id: Ib8729a985d2ed14ec062f98a9a8bcebacf90f8fc
Signed-off-by: bibhayr <bibhayr@nvidia.com>
Reviewed-on: http://git-master/r/440127
GVS: Gerrit_Virtual_Submit
Reviewed-by: Todd Poynter <tpoynter@nvidia.com>
Tested-by: Todd Poynter <tpoynter@nvidia.com>