dev-amdgpu: Remove unused psp_ring_retval integer

This change addresses the compiler failures that have been
causing any GCN3_X86 build to fail.
https://jenkins.gem5.org/job/compiler-checks/589/

Change-Id: Ifd8e2ef89549752ca4aedf0bc9fa47e831a822d3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/70217
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Matthew Poremba <matthew.poremba@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/dev/amdgpu/amdgpu_nbio.hh

@@ -109,7 +109,6 @@ class AMDGPUNbio
     Addr psp_ring_dev_addr = 0;
     Addr psp_ring_listen_addr = 0;
     int psp_ring_size = 0;
-    int psp_ring_retval = 0;
     int psp_ring_value = 0;
 };