rtl(axi_dw_downsizer): hold BRESP stable while BVALID asserted

[devtyagi3909]

Fixes #289.

The AXI specification requires that BRESP does not change value while BVALID is asserted and BREADY has not yet been seen. The downsizer violated this because its AW/W channel FSM (in W_IDLE) could preemptively reset the shared burst_resp accumulator before the slow B channel had finished its wait state, thereby corrupting the output response on subsequent cycles.

This fix ensures that:

• BRESP output is held stable during the BVALID wait state.
• The burst response accumulator is initialized at reset, and reset strictly according to B channel handshakes.
• The idle reset in the W channel FSM has been removed, completely decoupling the response accumulation from W channel boundaries.