opal_mpipl_query_tag() takes a pointer to a 64-bit value, which firmware
writes a value to. As OPAL is traditionally big endian this value will
be big endian.

This can be confirmed by looking at the implementation in skiboot:

  static uint64_t opal_mpipl_query_tag(enum opal_mpipl_tags tag, __be64 *tag_val)
  {
  	...
  	*tag_val = cpu_to_be64(opal_mpipl_tags[tag]);
  	return OPAL_SUCCESS;
  }

Fix the declaration to annotate that the value is big endian.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210421125402.1955013-2-mpe@ellerman.id.au