Use a separate function to look for reservation
chanctx. For multi-interface/channel reservation
search sematics differ slightly.

The new routine allows reservations to be merged
with chanctx that are already reserved by other
interface(s).
Signed-off-by: NMichal Kazior <michal.kazior@tieto.com>
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>