Bluetooth: Silent sparse warnings

Silence warnings below:

net/bluetooth/l2cap_core.c:1662:24: warning: Using plain integer
as NULL pointer
net/bluetooth/l2cap_core.c:1662:27: warning: Using plain integer
as NULL pointer
net/bluetooth/l2cap_core.c:1683:24: warning: Using plain integer
as NULL pointer
net/bluetooth/l2cap_core.c:1683:27: warning: Using plain integer
as NULL pointer
net/bluetooth/l2cap_core.c:2260:46: warning: Using plain integer
as NULL pointer
net/bluetooth/l2cap_core.c:2574:33: warning: Using plain integer
as NULL pointer
net/bluetooth/l2cap_core.c:2581:33: warning: Using plain integer
as NULL pointer
net/bluetooth/l2cap_core.c:4556:24: warning: Using plain integer
as NULL pointer
net/bluetooth/l2cap_core.c:4556:27: warning: Using plain integer
as NULL pointer

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>