Bluetooth: host: Fix L2CAP reconfigure response with invalid MTU

TSE18813 clarified IUT behavior and rejecting reconfiguration which
would result in MTU decrease is enough. There is no need to disconnect
L2CAP channel(s).

This was affecting L2CAP/ECFC/BI-03-C qualification test case
(TCRL 2022-2).

Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>
(cherry picked from commit 266394de)