Bluetooth: ATT: Set correct MTU for EATT L2CAP channel

The EATT MTU in Zephyr is static. We know it at channel creation time,
so we should communicate the MTU as part of channel creation.

Side note: With this approach, it should no longer be neccessary or
useful to do a channel reconfigure.

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