Bluetooth: Add missing selection of CONFIG_CRC16 for L2CAP layer

Fix net/bluetooth/l2cap.c build errors:

l2cap.c:(.text+0x126035): undefined reference to `crc16'
l2cap.c:(.text+0x126323): undefined reference to `crc16'
l2cap.c:(.text+0x12668e): undefined reference to `crc16'
l2cap.c:(.text+0x12683b): undefined reference to `crc16'
l2cap.c:(.text+0x126956): undefined reference to `crc16'
net/built-in.o:l2cap.c:(.text+0x129041): more undefined references to `crc16' follow

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>