Bluetooth: hidp: safely acquire hci connection

Claim device lock to safely enumerate hci connection list and bump
hci connection proxy device ref count simultaneously.

Also it prevents kernel crash due to null pointer
conn->hdev in deleting conn entry from sysfs.

CRs-Fixed: 446403
Change-Id: I5ec4283d359f95e47711dff171d0619b15263349
Signed-off-by: Sumit Bajpai <sbajpai@codeaurora.org>
Signed-off-by: Mallikarjuna GB <gbmalli@codeaurora.org>