Bluetooth: ATT: Introduce `bt_att_mtu`

This is a refactoring, only visible inside `att.c`.

Give the expression `chan->chan.tx.mtu` the name `bt_att_mtu`. Use it
when the intention is to get the ATT MTU property of a channel.

This is done in preparation a more complex expression for `bt_att_mtu`,
since the expression currently incorrect. The fix will come in a later
commit in the same PR.

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