drm/amdgpu: Drop unreachable return in amdgpu_reg_get_smn_base64()

drm/amdgpu: Drop unreachable return in amdgpu_reg_get_smn_base64()

amdgpu_reg_get_smn_base64() returns from all control-flow paths inside
the !adev->reg.smn.get_smn_base fallback path.

For version == 1, the function returns the base address from
amdgpu_reg_smn_v1_0_get_base(). For all other versions, the default
switch branch emits a dev_err_once() and returns 0.

The trailing return 0 after the switch is therefore unreachable and is
reported by Smatch as dead code:

  drivers/gpu/drm/amd/amdgpu/amdgpu_reg_access.c:317
  amdgpu_reg_get_smn_base64() warn: ignoring unreachable code

Remove the redundant return statement.

Fixes: 467ebfe65f6e ("drm/amdgpu: Add smn callbacks to register block")
Cc: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Lijo Lazar <lijo.lazar@amd.com>
Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>