efi/x86: libstub: Fix typo in __efi64_argmap* name

The actual name of the DXE services function used
is set_memory_space_attributes(), not set_memory_space_descriptor().

Change EFI mixed mode helper macro name to match the function name.

Fixes: 31f1a0ed ("efi/x86: libstub: Make DXE calls mixed mode safe")
Signed-off-by: Evgeniy Baskov <baskov@ispras.ru>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>