x86/efi: parse_efi_setup() build fix

In case without CONFIG_EFI, there will be below build error:

   arch/x86/built-in.o: In function `setup_arch':
  (.init.text+0x9dc): undefined reference to `parse_efi_setup'

Thus fix it by adding blank inline function in asm/efi.h
Also remove an unused declaration for variable efi_data_len.

Signed-off-by: Dave Young <dyoung@redhat.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>