WiredAccessoryManager: use dev index from UEventInfo to form devPath

DP extcon support does not always counts from index zero.
In checkDevIndex(), updating variable used to construct
dev path string, i.e. index, to dev_index so that
it also works in other device nodes.

CRs-Fixed: 2986549
Change-Id: Idb2ebfc18947681e9f8ca62bb8f7f5772b55b723