Fix crossbar locking up after reading to write-only address #85
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…tting out of sync
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When someone tries to read from a write-only region, it doesn't increment trans_count_reg (because it never sets trans_start), but it still gets s_cpl_valid to indicate the end of the transaction (once the DECERR goes through) and tries to decrement the transaction count. This can cause trans_count_reg to overflow and peg the transaction count at the limit, causing the bus to stall.
fixes #84