miss_handler: Fix AMO AXI ID mapping

Created by: niwis

In #1361, the default number of MSHR ports was increasesed by one. This mapped AXI IDs for bypassed stores from previously 4'b1010 to 4'b1011, colliding with the AXI IDs for AMOs. Apart from performance implications, this may conflict with A7.4.4 of the AMBA AXI protocol spec K ("ID use for Atomic transactions").

Fix this by assigning AMOs the next free AXI ID.