Bluetooth: hci_sock: Reset cookie to zero in hci_sock_free_cookie()

Reset cookie value to 0 instead of 0xffffffff in hci_sock_free_cookie()
since:
0         :  means cookie has not been assigned yet
0xffffffff:  means cookie assignment failure

Also fix generating cookie failure with usage shown below:
hci_sock_gen_cookie(sk)   // generate cookie
hci_sock_free_cookie(sk)  // free cookie
hci_sock_gen_cookie(sk)   // Can't generate cookie any more

Signed-off-by: Zijun Hu <zijun.hu@oss.qualcomm.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>