Bluetooth: ATT: Refactor PDU handling

Mark each PDU type explicitly instead of using BT_ATT_OP_CMD_FLAG for
this. For simple "error response or not" selection the command flag is
sufficient, but to prepare for support for enforcing both request and
indication flow control it's useful to easily look up the type of the
PDU.

Additionally, refactor the handler lookup to make the flow a bit more
streamlined.

Change-Id: I575848356934b6d636dcda8d10b7e7fde1095355
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>