efi/x86: Simplify i386 efi_call_phys() firmware call wrapper

The variadic efi_call_phys() wrapper that exists on i386 was
originally created to call into any EFI firmware runtime service,
but in practice, we only use it once, to call SetVirtualAddressMap()
during early boot.
The flexibility provided by the variadic nature also makes it
type unsafe, and makes the assembler code more complicated than
needed, since it has to deal with an unknown number of arguments
living on the stack.

So clean this up, by renaming the helper to efi_call_svam(), and
dropping the unneeded complexity. Let's also drop the reference
to the efi_phys struct and grab the address from the EFI system
table directly.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Matthew Garrett <mjg59@google.com>
Cc: linux-efi@vger.kernel.org
Link: https://lkml.kernel.org/r/20200103113953.9571-9-ardb@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>