kernel/mem: Recognize some deprecated memory port configs.
kernel/mem: Recognize some deprecated memory port configs.

Transparency is meaningless for asynchronous ports, so we assume
transparent == false to simplify the code in this case.  Likewise,
enable is meaningless, and we assume it is const-1.  However,
turns out that nMigen emits the former, and Verilog frontend emits
the latter, so squash these issues when ingesting a $memrd cell.

Fixes #2811.

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