smbios: Move smbios_write_type to the C file

This type is not used outside the smbios.c file so there is no need for it
to be in the header file. Move it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>