bluetooth: Fix warnings in l2cap_core.c

net/bluetooth/l2cap_core.c: In function ‘l2cap_recv_frame’:
net/bluetooth/l2cap_core.c:3758:15: warning: ‘sk’ may be used uninitialized in this function
net/bluetooth/l2cap_core.c:3758:15: note: ‘sk’ was declared here
net/bluetooth/l2cap_core.c:3791:15: warning: ‘sk’ may be used uninitialized in this function
net/bluetooth/l2cap_core.c:3791:15: note: ‘sk’ was declared here

Signed-off-by: David S. Miller <davem@davemloft.net>