L2CAP: Allowed mode is equivalent to preferred mode

In all use cases, we want one BASIC or ERTM exculsively, but we never
allow "both are ok" mode.

Remove allowed_mode.  Use preferred_mode to denote the desired mode.

Bug: 159815595
Tag: #refactor
Test: compile & verify basic functions working
Change-Id: I846eca1912fc9ea04dfa57c5698039d62e634186