efi_memory: return MapKey

efi_get_memory_map should set a defined value for map_key.

We later can introduce the test against this value in
efi_exit_boot_services as required by the UEFI standard.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>