Bluetooth: Host: l2cap: disconnect when PDU > MPS

The Bluetooth spec states that a K-frame exceeding the device's MPS
should trigger a disconnection of the channel.

Fixes two PTS tests that verify this behavior:
L2CAP/LE/CFC/BV-27-C
L2CAP/ECFC/BV-34-C

The current behavior is a silent truncation, which is not very nice for
the user, even if the spec would allow it.

Fixes #52228 and #51174.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>