bluetooth: Fix use-before-initiailized var.

net/bluetooth/l2cap_core.c: In function ‘l2cap_recv_frame’:
net/bluetooth/l2cap_core.c:3612:15: warning: ‘sk’ may be used uninitialized in this function
net/bluetooth/l2cap_core.c:3612:15: note: ‘sk’ was declared here

Actually the problem is in the inline function l2cap_data_channel(), we
branch to the label 'done' which tests 'sk' before we set it to anything.

Initialize it to NULL to fix this.

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