efistub/smbios: Simplify SMBIOS enumeration API

Update the efi_get_smbios_string() macro to take a pointer to the entire
record struct rather than the header. This removes the need to pass the
type explicitly, as it can be inferred from the typed pointer. Also,
drop 'type' from the prototype of __efi_get_smbios_string(), as it is
never referenced.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>