TDLS: Use existing peer entry if available when processing discovery

TDLS: Use existing peer entry if available when processing discovery

Peer entries were getting added on every discover request from the peer,
thus resulting in multiple entries with the same MAC address. Ensures
that a check is done for the presence of the peer entry and reuse the
existing entry instead of adding a new one.

Signed-hostap: Jouni Malinen <jouni@qca.qualcomm.com>