efi/ia64: Use local variable for EFI system table address

The IA64 code never refers to the EFI system table except from
inside the scope of efi_init(). So let's use a local variable
instead of efi.systab, which will be going away soon.

Tested-by: Tony Luck <tony.luck@intel.com> # arch/ia64
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>