ice40: Fix SPRAM model to keep data stable if chipselect is low
ice40: Fix SPRAM model to keep data stable if chipselect is low

According to the official simulation model, and also cross-checked
on real hardware, the data output of the SPRAM when chipselect is
low is kept stable. It doesn't go undefined.

Signed-off-by: Sylvain Munaut <349f318521b45672c77207abf1a5ed60eb011db1@246tNt.com>

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