Bluetooth: Make L2CAP chan_add functions static

Remove sparse warnings below:

...
net/bluetooth/l2cap_core.c:302:6: warning: symbol '__l2cap_chan_add' was
not declared. Should it be static?
net/bluetooth/l2cap_core.c:351:6: warning: symbol 'l2cap_chan_add' was
not declared. Should it be static?
...

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo@padovan.org>