Bluetooth: hidp: safely acquire hci connection

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

This patch incorporates David Herrmann's fix to prevent adding an
HID device when the hci connection no longer exists.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>