Bluetooth: Remove special ATT data channel handling

Now that we've got the fixed channel infrastructure cleaned up in a
generic way there's no longer a need to have a dedicated function for
handling data on the ATT channel. Instead the generic
l2cap_data_channel() handler will be able to do the exact same thing.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>