musa: fix failures in test-backend-ops for mul_mat_id op

[yeahdongcn]

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Ref: #15131

Testing Done

ToT:

root@xiaodongye-s80:/ws# ./build/bin/test-backend-ops  | grep FAIL
ggml_cuda_init: GGML_CUDA_FORCE_MMQ:    no
ggml_cuda_init: GGML_CUDA_FORCE_CUBLAS: no
ggml_cuda_init: found 1 MUSA devices:
  Device 0: MTT S80, compute capability 2.1, VMM: yes
[MUL_MAT_ID] NMSE = 0.473073968 > 0.000500000   MUL_MAT_ID(type_a=f32,type_b=f32,n_mats=4,n_used=1,b=0,m=512,n=32,k=256): FAIL
[MUL_MAT_ID] NMSE = 0.602911949 > 0.000500000   MUL_MAT_ID(type_a=f32,type_b=f32,n_mats=4,n_used=1,b=1,m=512,n=32,k=256): FAIL
[MUL_MAT_ID] NMSE = 0.801315053 > 0.000500000   MUL_MAT_ID(type_a=f32,type_b=f32,n_mats=4,n_used=2,b=0,m=512,n=32,k=256): FAIL
[MUL_MAT_ID] NMSE = 2.501388765 > 0.000500000   MUL_MAT_ID(type_a=f32,type_b=f32,n_mats=4,n_used=2,b=1,m=512,n=32,k=256): FAIL
[MUL_MAT_ID] NMSE = 1.142949490 > 0.000500000   MUL_MAT_ID(type_a=f32,type_b=f32,n_mats=8,n_used=1,b=0,m=512,n=129,k=256): FAIL
[MUL_MAT_ID] NMSE = 0.820664836 > 0.000500000   MUL_MAT_ID(type_a=f32,type_b=f32,n_mats=8,n_used=1,b=1,m=512,n=129,k=256): FAIL
[MUL_MAT_ID] NMSE = 0.730463410 > 0.000500000   MUL_MAT_ID(type_a=f32,type_b=f32,n_mats=8,n_used=2,b=0,m=512,n=32,k=256): FAIL
[MUL_MAT_ID] NMSE = 1.249068448 > 0.000500000   MUL_MAT_ID(type_a=f32,type_b=f32,n_mats=8,n_used=2,b=1,m=512,n=32,k=256): FAIL
[MUL_MAT_ID] NMSE = 1.097372514 > 0.000500000   MUL_MAT_ID(type_a=f32,type_b=f32,n_mats=8,n_used=4,b=0,m=512,n=32,k=256): FAIL
[MUL_MAT_ID] NMSE = 0.734338886 > 0.000500000   MUL_MAT_ID(type_a=f32,type_b=f32,n_mats=8,n_used=4,b=1,m=512,n=32,k=256): FAIL
  Backend MUSA0: FAIL
FAIL

With this fix:

root@xiaodongye-s80:/ws# ./build/bin/test-backend-ops | grep FAIL
ggml_cuda_init: GGML_CUDA_FORCE_MMQ:    no
ggml_cuda_init: GGML_CUDA_FORCE_CUBLAS: no
ggml_cuda_init: found 1 MUSA devices:
  Device 0: MTT S80, compute capability 2.1, VMM: yes

[JohannesGaessler]

Github doesn't let me make the correct suggestion, but please apply the same fix to cp_async_available from which I copied the logic (and where it seems the defect doesn't manifest as a bug).