drm/amdgpu: use u32 for buf size in __amdgpu_eeprom_xfer

And also make sure the value of msg[1].len should be in the range of u16.

Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>