[flash_ctrl] Fix read buffer re-allocation for even buffers

Previously, the read buffer re-allocation arbiter erroneously did not
factor in the `ack_i` signal coming from the Flash macro. While the open
source simulation model never really de-asserts `ack_i`, some real
Flash macros may do so. As a result, `req_o` might be high for multiple
clock cycles and the read buffer re-allocation may skip even read
buffers. This effectively halves the number of usable read buffers which
may have a negative impact on performance.

The fix is easy and just involves factoring in `ack_i` to take the
actual Flash macro request handshake for doing allocations.

This resolves #23797.

Signed-off-by: Pirmin Vogel <[email protected]>

Author: vogelpi

hw/ip_templates/flash_ctrl/rtl/flash_phy_rd.sv

@@ -179,7 +179,7 @@ module flash_phy_rd
     .idx_o(),
     .valid_o(),
     .data_o(),
-    .ready_i(req_o & no_match)
+    .ready_i(req_o & ack_i & no_match)
   );
 
   // which buffer to allocate upon a new transaction

hw/top_earlgrey/ip_autogen/flash_ctrl/rtl/flash_phy_rd.sv

@@ -179,7 +179,7 @@ module flash_phy_rd
     .idx_o(),
     .valid_o(),
     .data_o(),
-    .ready_i(req_o & no_match)
+    .ready_i(req_o & ack_i & no_match)
   );
 
   // which buffer to allocate upon a new transaction