drm/amdgpu: fix uninitialized scalar variable warning

[ Upstream commit 9a5f15d2

 ]

Clear warning that uses uninitialized value fw_size.

Signed-off-by: Tim Huang <Tim.Huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>