efi: Use the correct GUID for the SMBIOS table

EFI does not use the 'anchor string' to determine the SMBIOS table
version, instead preferring to have two separate GUIDs. Use the correct
one, depending on the table version.

Call unmap_system() to balance to the use of map_sysmem()

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>