Bluetooth: Rename l2cap.c to l2cap_core.c

In a preparation to the the L2CAP code split in many files.
Signed-off-by: NGustavo F. Padovan <padovan@profusion.mobi>

net/bluetooth/Makefile

@@ -11,3 +11,4 @@ obj-$(CONFIG_BT_CMTP)	+= cmtp/
 obj-$(CONFIG_BT_HIDP)	+= hidp/
 
 bluetooth-y := af_bluetooth.o hci_core.o hci_conn.o hci_event.o mgmt.o hci_sock.o hci_sysfs.o lib.o
+l2cap-y := l2cap_core.o