drm/amdgpu/sdma: add missing release_firmware() in amdgpu_sdma_init_microcode()

In some error path in amdgpu_sdma_init_microcode(), release_firmware() is
not called, the memory allocated in request_firmware() will be leaked,
calling amdgpu_sdma_destroy_inst_ctx() which calls release_firmware() to
avoid memory leak.

Fixes: 15aa1305 ("drm/amdgpu: add function to init SDMA microcode")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>