drm/amdgpu: Fix truncation issues in smu_v13_0_init_microcode

Fixes the below with gcc W=1:
drivers/gpu/drm/amd/amdgpu/../pm/swsmu/smu13/smu_v13_0.c: In function ‘smu_v13_0_init_microcode’:
drivers/gpu/drm/amd/amdgpu/../pm/swsmu/smu13/smu_v13_0.c:108:52: warning: ‘%s’ directive output may be truncated writing up to 29 bytes into a region of size 23 [-Wformat-truncation=]
  108 |         snprintf(fw_name, sizeof(fw_name), "amdgpu/%s.bin", ucode_prefix);
      |                                                    ^~       ~~~~~~~~~~~~
drivers/gpu/drm/amd/amdgpu/../pm/swsmu/smu13/smu_v13_0.c:108:9: note: ‘snprintf’ output between 12 and 41 bytes into a destination of size 30
  108 |         snprintf(fw_name, sizeof(fw_name), "amdgpu/%s.bin", ucode_prefix);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Suggested-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>