Bluetooth: Correct `bt_att_create_rsp_pdu` reservation when EATT is enabled

When EATT is enabled, bt_att_create_rsp_pdu used to reserve headroom for
the SDU header even when responding on the UATT bearer.

That subtracted from the room for the ATT payload in the buffers. The
remaining buffer size was insufficient to create a PDU of ATT MTU size,
since the exchanged local MTU is calculated the with the assumption that
the SDU header is not present.

This broke the ATT MTU promise, and e.g. our read response will have two
bytes fewer than promised. This caused a failure in PTS.

The new bt_att_create_rsp_pdu pays attention to the bearer type and only
allocates the SDU header on EATT bearers.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>