Bluetooth: Host: Fix GATT Long Read for EATT

When EATT is established, the value returned from `bt_att_get_mtu` is
not useful to determine the ATT_MTU that applies to a ATT response. This
is because `bt_att_get_mtu` may return the value for a different bearer
than the request is serviced on.

To fix this, the params struct for the GATT read operation is given a
new field that will record the ATT_MTU that applies to this ATT
operation. This value is then used to determine if the GATT long read
operation is concluded.

Fixes: https://github.com/zephyrproject-rtos/zephyr/issues/61741



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