drm/amdgpu: update fw_load_type module parameter doc to match code

amdgpu_ucode_get_load_type() does not interpret this parameter as
documented.  It is ignored for many ASIC types (which presumably
only support one load_type), and when not ignored it is only used
to force direct loading instead of PSP loading.  SMU loading is
only available for ASICs for which the parameter is ignored.

Signed-off-by: Yann Dirson <ydirson@free.fr>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>