nimble/l2cap: Fix searching channel by dcid
nimble/l2cap: Fix searching channel by dcid

Source CIDs are ordered in the list, but dcid not, therefore when
searching by dcid we cannot assume the order.

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble