Bluetooth : hidp: Get the valid hci_conn while unlink

Since the context where unlink will be called is unaware of
validity of hci_conn pointer, fetch the valid hci_conn
before unlinking.

CRs-Fixed: 433361
Change-Id: Ief71997a3f007ea43431765c6cf543ab0d6416a8
Signed-off-by: Sumit Bajpai <sbajpai@codeaurora.org>