Floss: Fix reconfigure L2CAP mode in receiving failure response

In the original code, we check if basic mode is supported by checking
preferred_mode. This is wrong and unnecessary because preferred_mode
only stores one mode and the basic mode should always be supported.

This CL removes the checking entirely.

Bug: 270529845
Test: run L2CAP/CMC/BV-03-C and L2CAP/CMC/BV-07-C
Change-Id: I369bcf9e63cb8ffe52eceeb7a18c1cfa17e8a052